The heart of what a spellcaster brings to any Group is their spells. They determine most of the play style and focus for a given Discipline, but also can represent a significant task to wade through and weighing the various options. The goal of this post is to analyze the spell selections in the first five Circles and give some idea of how those options may inform the emphasis of the Discipline, play style, as well the relative use of a particular spell. The reason this will only address the first five Circles is that you get to pick spells at those Circles - starting at Sixth Circle, you are on your own and it is largely in your GM's hands.
Depending on the Optional Rules used in your game, how you address picking up new spells will change. If spells can be purchased and do not cost Legend Points, there is little reason to not learn everything you can get your fingers on. If they do cost Legend Points, you may want to be more selective of what you include in your grimoire.
Astral Spear, Spirit Dart and Spirit Grip: These are your first combat spells and are going to be your best offensive options for a very long time. Learning how to capitalize on each of these throughout your career is going to make all the difference in how much you are able to impact a conflict with damage. The key to an effective strategy is knowledge. While they all target Spell Defense, there are other differences that make each of them more useful in a particular scenario.
Astral Spear has good damage (WIL+6), affects Mystic Armor and good range, but has a Thread; Spirit Dart has low damage (WIL+2), affects Mystic Armor and range, with no Threads; and Spirit Grip has good damage (WIL+6), affects Physical Armor and no range, with no Threads. In general, Mystic Armor tends to be lower, but not always. Knowing the Armor values is the most important thing to making the most of these spells; a high Mystic Armor (5+) may benefit most from Astral Spear or Spirit Grip (Depending on Physical Armor), while a low Mystic Armor would be most vulnerable to the constant attack with Spirit Dart. Right up until you get two Enhanced Matrices, at which point it is all Astral Spear, all day.
Bone Dance: A powerful control spell that can effectively remove a single target from combat, or at least make them an easier target. If their are environmental dangers, such as a cliff or Death's Sea, then there is significantly added danger.
Chilling Circle: Two threads, low damage and low area. Pass.
Command Nightflyer: Fun, but not terribly useful yet. There are later spells that will build on this one to expand its uses.
Detect Undead: To be honest, this spell generally seems like a waste of a Spell Matrix. If you have it around, you probably already suspect there are undead. When you most likely want it is when it probably isn't already in a Matrix.
Dry and Wet: If there is no cost associated with learning this spell, it's neat in ways that probably are not going to be useful outside of that one time it was awesome to have.
Experience Death: One of the best investigative powers in the game, hands down. There is a significant downside: you take the damage that killed someone, with the Effect Test offering a buffer. If your campaign is going to have any sort of mysteries surrounding corpses, this will be useful. Just make sure someone is around that can shove a healing potion down your throat.
Insect Repellent: See Dry and Wet above.
Mount Scare: It requires a Thread, but if you know you are going to encounter cavalry, such as scorchers, then this can effectively remove them from the fight. Outside of that, it's useless.
Putrefy: See Dry and Wet above.
Undead Struggle: This locks down a single undead target and doesn't have any Threads. The odds of coming out the winning party are stacked in the Nethermancer's favor. This is good whenever there are powerful undead about, regardless of your Circle. Not as useful against hordes of them.
Bone Circle: The is another spell that becomes more useful as you gain Circles and have additional ways to use it. It forms the basis of some of the more powerful Nethermacy.
Detect Nethermancy Magic: Wizards are able to just Detect Magic and this spell rarely sees much play. Since Astral Sight is a Discipline Talent for Nethermancers, when Detect Nethermancy Magic is applicable and the Astral Sight is not is something of a grey to non-existent area.
Ethereal Darkness: This is an interesting spell that blinds everyone except anyone with Nethermancy. Your Group may be very upset if you use this constantly and there are a considerable number of Horrors out there with Nethermancy. It also requires a Thread, which is often problematic in combat.
Fog Ghost: You summon a minion to do combat for you. Unfortunately, they aren't very powerful for two Threads. You would likely be better off supporting the Group in other ways. Also, they may attack your "friends".
Gadfly: One Thread and you can render a target Harried. This is particularly effective in encounters with small numbers of powerful opponents.
Life Circle of One: This seems useful, but in general it takes a certain amount of desperation for it to gain value. Also time - this spell has two Threads and may take a number of them to keep safe. It can be great for a dramatic moment, but not so much for regular use.
Pocket Guardian: It mostly prevents pickpocketing. This may or may not be a thing for you.
Repel Animal: See Dry and Wet above.
Shadow's Whisper: If you are in a game with intrigue, this is an excellent spell. If you are raiding kaers and/or spend all of your time in the wilderness, not so much.
Shield Mist: Every Nethermancer should have this spell. It should also be the first one to get an Enhanced Matrix. Seriously - this will save your life.
Dark Messenger: One of the spells that makes use of Command Nightflyer to deliver relatively short range messages. It's fun and will likely find some use in the right circumstances - spells that can be used outside of combat benefit from the ability to attune them as needed, which greatly increases their usefulness since they aren't occupying a valuable Matrix when unneeded.
Death Trance: This is a spell infinitely more likely to be used by an NPC than a PC.
Death's Head: The Nethermancer has a number of powerful control style effects that remove opponents from the area. This particular one has a Thread, but can be used each turn as a Simple action over a duration (Spellcasting Rank + 5 Rounds), making it excellent for crowd control and one of the two most useful spells for this Circle. This will not affect things immune to fear.
Dispel Nethermancy Magic: Similar to Detect Nethermancy Magic above, but likely to see significantly more use. Definitely a useful spell to have in your arsenal, but not the must have for this Circle.
Fog of Fear: Very similar to Death's Head above, this has two Threads and affects everyone (including allies) in the area immediately. It is great for clearing the vicinity, but lacks that critical "control" element which makes these kind of control effects so useful. While it has its uses, there are significantly better options.
Grave Message: One of the spells that makes use of a Bone Circle, this allows Nethermancer pen-pals to stay in touch. If you don't have a Nethermancer you want to send messages to over a distance that regularly hangs out at a specific Bone Circle, cemetery, or location that spirits are active, you won't get much use from this spell.
Pack Bags: See Dry and Wet above.
Preserve: Along the same lines as Dry and Wet above, this spell often sees considerable use in my games because my players end up playing with... things that decay. It's fun and certainly worth learning, but there are definitely some better choices to go after first.
Shadow Meld: With two Threads and a duration of Spellcasting minutes, this spell can be difficult to use as an escape, or in support of subterfuge. Generally, this is a great way for things to go wrong. For some, that may be a feature.
Spirit Double: Ever wonder what happens when you stick your arm in that hole, but know it probably is going to turn out poorly? If those kinds of thoughts are commonplace for you, then this is your spell! This spell is essentially an expendable humanoid. If you have use for such a thing, it would be worth getting this spell at some point.
Summon Bone Ghost: Another spell that makes use of a Bone Circle. If your campaign spends most of its time in a particular area where you have an established Bone Circle, there may be some use for this spell. If you are a more typical adventurer and spend most of your time as a vagabond, then it is unlikely you will find much use here.
Animate Skeleton: Ever wish that you had a group of skeletons at your command for a limited time? This will get you there. It has two Threads, must be cast where there are skeletons to animate in the first place, they don't last for very long, and are pretty weak, but you will have skeletons. All said, it's not that useful, but there is no denying the panache. Some Namegivers look poorly at their dead being animated, just sayin'.
Astral Flare: Two Threads is steep, but this is a great way to stick it to Horrors. It is common for them to perceive both physical and astral space at the same time. Outside of that, it is likely to be of limited use - but that is a helluva use.
Blood Servitor: This one of those fun spells that has little to no use in combat. It has a good duration (Spellcasting Rank hours) and range (Spellcasting Rank miles), and creates a small flying minion that will do whatever you want. You can even perceive what they perceive! The only downside is your commands have to be simple, with no more words than your Spellcasting Rank. This is one of my favorite spells.
Dark Spy: Another spell that expands on Command Nightflyer, this allows you to see through its eyes. While more subtle than Blood Servitor and the various effects that can build on the original are fun, I generally prefer Blood Servitor.
Fatal Food: Most useful for NPCs, there is still some value in intrigue games where the PCs are not terribly nice people.
Friend or Foe: This peculiar spell is most useful when you want to stop everyone involved from fighting, unless they don't care much who they are killing, so long as they are killing. The narrow application of this spell makes it unlikely to see any use.
Last Chance: Let's face it, this is the best spell this Circle, if not one of the best spells period. You are going to want this, and you are going to use this.
Nightflyer's Cloak: Yet another spell that expands on Command Nightflyer. This is great for the Nethermancer that is often irritated by the constraints of Command Nightflyer and wants to get in there on their own. Good for scouting and also has the amusing possibility of the Nethermancer being commanded by Command Nightflyer as well. Every Nethermancer that takes this spell has to know it is going to happen at least once.
Spirit Servant: While this is probably not useful in the strictest sense, this spell is endearing in a way that is hard to articulate unless you have had a faithful spirit butler Named Jeeves. It is so thematic for a Nethermancer, that I cannot actually think of playing one without this spell. That being said, there are a couple of spells from this Circle I would get first.
Viewpoint: This is a great spell for gathering information, and a must have for any game that features any amount of intrigue. It allows you to create a one-way "window" through a wall. There is almost certainly a situation where that will be useful.
Visions of Death: Similar to Pain above, this spell has one Thread, but does not require concentration to keep up. A big downside is that it is a fear effect, instead of a pain effect, and there are considerably more ways to get bonuses to that, or be outright immune.
Animate Spirit Object: Sorcerer's Apprentice, anyone? With a good duration (Spellcasting Rank x 10 minutes) and considerable versatility, this is a handy spell to have around for "just in case". Be careful, you GM is given some latitude to mess with you as the spirit is capricious and mischievous.
Astral Horror: Illusion Nethermancy spells are rare, which makes them more effective in general, and this summons a fairly powerful (if illusory) monster to fight for you. It has three Threads, which makes this an investment to cast and useful only in the most protracted conflicts (or where you have a few rounds to prepare). While there is nothing strictly wrong with it, there are probably more useful spells in general to take.
Astral Mount: A very useful spell for travelling. It can get your entire Group mounted without worrying about what happens to the mount when you leave it alone (typically: gets eaten). There is considerable upside to this spell.
Astral Whisper: This spell is an invitation for the GM to screw with you and there is little obvious benefit. Eavesdropping on a Horror? Best case scenario, you find out who it is taking to the prom or plans to torture first. Worst case scenario, you are driven insane by what you hear. There is probably a good use for this, but as a GM, I kinda hope my player takes this because it seems like fun. For me.
Blind: A powerful debuff that does exactly as advertised and is remarkably difficult to counter. With three Threads, it had better be; this is also the biggest downside to this spell. Another benefit is that this is more likely to be effective against Horrors than the other control options, but by no means a guarantee (they have to have eyes, after all). If you have gotten a lot of mileage from the numerous control options available to you, this may be a good addition to your arsenal. If you only have Death's Head or Pain, I would suggest getting the other one before this.
Circle of Astral Protection: To be honest, I'm not a fan of any effect that limits your mobility. This is especially true since most attacks that target Mystic Armor are also ranged and this neutralizes any melee retaliation from the PCs. At three Threads, this also requires some advance planning to put into place.
Incessant Talking: Truly a strange spell that isn't subtle enough to be really useful in social situations, since you have to babble along. It's biggest value is to prevent another spellcaster from casting spells that have a verbal component, but there are a lot of spells without that. Also, other magicians are unlikely to have much trouble resisting the effects. At three Threads... if you are willing to look like a fool to make someone else look like a fool?
Pass Ward: An interesting spell that is most likely to see use from an NPC, but it certainly isn't out of the question for a PC to make use of it. The biggest downside to this spell is that the damage isn't going to be particularly significant to determined enemies of a similar Circle and the ward goes away after being triggered.
Sculpt Darkness: Groups that spend time laying in wait will find this useful, otherwise this is more likely to be used against the PCs. It is pretty fun and gives some nice bonuses to hiding, but is of limited broad application.
Sense Horror: Since Horrors are almost certainly a part of any Earthdawn game, this is likely to be useful. In a traditional exploration game, the presence of a Horror probably won't need this kind of confirmation, but in a setting with intrigue, this would be very useful.
Shadow Hunter: Useful if an enemy has escaped and you have one of their possessions. While they are competent in a fight, they aren't that powerful. For best results, try and follow them to your quarry.
Spiritual Guidance: Fun, but be prepared for a trip. It is also worth noting that not all GM's like to be put on the spot and may respond best when this is used in-between games.
Star Shower: This seems powerful on paper, but it requires a considerable amount of coordination to setup. The physical damage is a small downside, but the fact it affects all of your companions in the radius is a thing. You are probably in a bad way if you need to use this.
Target Portal: For four Threads, you can increase the range of a non-touch spell by 1000 yards. That is still a lot of Threads.
Tears of the Scourge: There are not many better spells for incapacitating a group of targets, though you and your allies cannot harm them when they are incapacitated with this spell. This is best when used to end a fight where you don't want to kill your foes, but they are making that a very difficult proposition.
Whisper Through the Night: Any ability that can gather information is a valuable one, and this is no exception. This is one of my favorite (and the most useful, imho) spells from this Circle.
Wither Limb: This is a very powerful spell and approaching permanently debilitating if it causes a Wound (which is rather likely given the WIL+8 Effect Test). When any side starts to deploy this spell, things have gotten ugly and were probably already personal. If they weren't before, they are now. Don't expect this to end well.