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.
Assuring Touch: No Threads, a decent duration (in rounds) and a small bonus to resist fear effects. By no means amazing, but this is an entirely reasonable spell by any stretch of the imagination.
Bellow of the Thundras: Not game changing, but if this is our Independence Day, or perhaps an apocalypse needs to be cancelled, accept no substitute.
Best Face: While the duration may be problematic (minutes), there are a lot of ways this spell can prove to be remarkably useful. It aligns nicely with how these adepts generally go about solving problems.
Blazing Fists of Rage/True Blazing Fists of Rage: This is the first entry into the shell game that is combat with an Illusionist. Each damage spell has a illusion version and an true version. The illusion potentially does more damage, but can be resisted by sensing the illusion. While treating the true version as an illusion will pretty much just hurt. It's very interesting, but you need to be interested in playing it in the first place. I would highly recommend getting a couple playing cards and writing "Illusion" on one, "True" on the other. Assuming you have both versions of the spell, when casting, place one face down. Have the GM determine how the target will react, then reveal the nature of the spell..
All of that being said, this is a probably going to be a useless spell. It is a buff, rather than a directly damaging spell, that requires the target to attack with Unarmed Combat. Low Circle Beastmasters (particularly windlings) and Purifiers may get something out of this, but otherwise just steer clear.
Catseyes: You can give anyone low-light vision for a few minutes. This can be remarkably useful in the right situation.
Disguise Metal: Let's be realistic, this spell makes money look like something other than money, or even more frequently it will make something made of metal look like it is made of gold. It isn't going to last very long (nor will it actually make it heavier), so be careful with how much you abuse this. Which is to say, make sure that you won't be around when the jig is up.
Displace Image: This is an interesting spell that is hindered by its limitations. It conceals your true location, but will still mimic all of your actions. In combat, it's biggest value seems to be setting up for a Surprise Strike, as maintaining the illusion under attack requires concentration. Outside of combat, where it has the most interesting potential, the duration is too short (rounds) to be of much actual use.
Light: It creates light. You can also use that light to dazzle a target, but with one Thread to do so, I hope there are better options are your disposal (at First Circle, there might not be).
Pauper's Purse: Here is a strange spell. It can be used to protect (some of) your money from theft, or to temporarily deprive the target of some funds (to whatever end). The problem with the former is the duration is in minutes and the latter is that you have to touch wherever the money is. Sorting out how to make that work to your advantage is what playing an Illusionist is all about.
Rope Guide: So you can concentrate to have a rope wriggle anywhere within 30 yards of you. There is likely going to be some point in the future where that will come in handy. Just be prepared for some potentially crass table talk about this spell at times.
Unseen Voices: This is a great spell, particularly for investigation, social or espionage driven games. There are a lot of ways to use this effect. This is a bread-and-butter spell for Illusionists.
Blindness: It requires a Thread and pretty much everything the target is going to do is a Sensing Test, but blinding a single target is wildly effective. As long as the target needs to see.
Crafty Thought: I can virtually guarantee that the first time this spell comes up, there is going to be a lot of reading and re-reading of the text, trying to sort out exactly what it does. Which is not a whole lot. It can give the suggestion of a simple image/action, but there is no impetus to follow through on it. If you are conducting psychological warfare on a particular person, this may be very useful (that isn't a joke).
Disaster: Any character that uses Surprise Attack regularly may find this to be a boon. Also, if you need to get a short head start on an escape.
Encrypt: This would be a great spell in an espionage-driven game if the duration was longer. As it is, it is useful only to hide something for a few hours. There are situations where it is still useful, but the timing may be pretty crucial.
Ephemeral Bolt/True Ephemeral Bolt: Here is the next entry into Illusionist combat spells, and these are significantly better than Blazing Fists of Rage. In fact, the illusion version of this spell is great. It has no Threads, good range and good damage. The downsides are the whole illusion thing and the fact it does physical damage. Nonetheless, get both of these.
Impossible Knot: This is another in the long list of strange Illusionist spells, though this one has a duration in days, which is a bonus. There are all kinds of strange ways creating a knot that is almost impossible to untie can be useful. If you're playing an Illusionist, I certainly hope that you enjoy trying to come up with them.
Innocent Activity: If your Group gets into illicit activity (the answer to this is almost certainly "yes"), this spell will almost certainly come in useful. The duration isn't great (rounds), but it is hopefully enough time to do something clearly illegal in plain sight and conceal that activity.
Monstrous Mantle: Two Threads and a so-so duration for this buff spell that can be disbelieved. However, it does give a variety of bonuses, which is very nice. The flavor of the spell is a little strange - I generally simply treat it as a glamour making the target appear significantly more impressive and dangerous, though if the original description works for you, to each their own.
Remove Shadow: A small bonus to anything related to being unseen with a duration in minutes isn't bad. It's not amazing, but sometimes anything can count.
Send Message: In anything other than a hardcore kaer-crawling game (in which case, why are you playing an Illusionist at all?), this is a very good spell. The limited duration can work for you, leaving no trace of the note and limited opportunity to directly copy it. This spell is highly recommended.
Tailor: This is a very useful spell for anyone that engages in social interaction. Anything to help improve Attitude is a boon. This can also potentially be used to make things rather difficult for someone you don't like. I think that's a twofer.
Weather Cloak: There is nothing particularly special about this spell. It's not terrible, but also not terribly interesting.
You Got Me: If you like Best Face, then consider this the upgrade to that spell - you will want it. Similarly, if Best Face had too many limitations for you, look into this version.
Alarm: Most useful as an early warning protecting an unguarded (or guarded, if you fear the guard is going to go out like a chump) position. Or if you want to publicly bust someone for going somewhere. Not so useful if you are doing the sneaking. The duration (minutes) prevents it from being useful for any extended time.
And Then I Woke Up: A significant bonus to detect an illusion. With the duration (2 rounds), it is really just a single illusion. Not bad, but nothing amazing to see here. It does reinforce the fact that the best foil to an Illusionist is another Illusionist - they know all of your tricks.
Blinding Glare: It can be powerful, but also rather difficult to make work. Essentially, you need targets that at range from your Group, but not too much range, because this spell only goes 12 yards. It has many of the same issues as Ethereal Darkness and Smoke Cloud, though the ability to use it at a distance makes it significantly better. Which moves it from "to be avoided" into "maybe".
Dampen Karma: Against a strong opponent (especially a Horror), this can be pretty amazing, particularly for only one Thread. This is a great spell to have in your arsenal.
Detect Illusionism Magic: Wizards are able to just Detect Magic and this spell rarely sees much play. Since Astral Sight is a popular (even vital) Talent Option for Illusionists, when Detect Illusionism Magic is applicable and Astral Sight is not is something of a grey to non-existent area.
Dispel Illusionism Magic: Similar to Detect Illusionism Magic, but likely to see significantly more use. A useful spell to have in your arsenal, but not the must have.
False Floor: Here is another example of a quintessential Illusionist spell. What exactly would you do with a spell that creates the impression that there is a trap? That is the kind of question that an Illusionist should enjoy grappling with.
Impossible Lock: Similar to Impossible Knot, but the duration is in hours. It's a reasonable way to keep something locked for a while, thwarting something you probably know about ahead of time, but not useful for general protection. It is worth mentioning that there has to be a lock and it only works against lock picking, not general attempts to open the door with a key.
Mind Fog: Yeah, this spell is just fun. Seriously fun. It is worth taking and putting in an Enhanced Matrix when you get one of those. A million and one uses. Easily one of the best spells in the game.
Nobody Here: Great to have for any Group that intends to get into trouble, or maybe sneak around a little. Which may as well define every Group, since forever.
Phantom Warrior: An entirely decent buff spell, giving a bonus to Physical Defense and against active defenses; which is pretty good for one Thread. The downside is that every action made against the character is pretty much a Sensing Test.
See the Unseen: Probably the best buff for perception Tests (as opposed to Perception Tests) around. It works in conjunction with other Talents (such as Astral Sight and Search), you just must be looking for something hidden.
Suffocation: Three Threads and a brutal ranged area attack that affects Mystic Armor. With a duration. The number of Threads mean this isn't going to be useful all of the time, but if you can get the drop on some adversaries, it will be brutal.
Aura: An interesting spell that generally seems to be most useful for illuminating a single target for a short time.
Bleeding Edge: This spell isn't interesting, it's downright weird. Creepy, and very useful if you want to creep people out, but not so useful as a damage buff considering the requirements - you, effectively, have to steal someone's weapon and then use it against them. If you can pull that off, why not just keep their weapon and use yours that will have all of your neat bonuses baked in?
Circle of Well Being: A healing effect for Illusionists. Probably the most fiddly, but definitely worth taking.
Clarion Call: Pretty much the Jedi mind trick. This is very powerful and you are going to want it
Conceal Tracks: An anti-tracking spell. It's pretty specific in application, but you never know when it might come in handy.
Eyes Have It: The good news is this blinding effect can selectively effect a different target each round, and it lasts until they succeed at a Willpower Test. Oh, and it lasts for minutes. The only downside is that this spell as three Threads. This is a good spell.
Hunger: This spell is pretty messed up. It can be effective at whatever long-term warfare you want to conduct against the target, but realize you are probably a terrible person for using this spell.
Improved Alarm: It's just like the original Alarm spell, but it can be discrete and the spellcaster has some control over its notifications. That makes this a significantly better spell than the original.
Memory Blank: Similar to Memory Fog, but it requires one more Thread (for two total). It has a much longer duration and can be used to make the target forget up to a few minutes. You are going to find ways to use this.
Multi-Missile: Requires two Threads, a successful Attack Test, the Avoid Blow Test counts as a Sensing Test, you have to be next to the target and the damage is terrible. Move along, nothing to see here.
Nightmare of Foreboding: If you learned Hunger, this is a nice companion to take with it. There are really too many Threads to make this useful in a combat situation.
Stop Right There: This is a great spell. I highly recommend learning it and always keeping it in a Spell Matrix. If you have to reattune it on the fly, it may already be too late.
Unmask: Not real great, unless you are deeply involved in some intricate plots involving illusionary identities. It does allow for some Scooby-Doo unmasking, however.
Awaken: Strange, but useful. It will heal someone so that the regain consciousness, which is the primary use of this spell (let's face it), along with some other applications along the way. Pretty useful.
Bond of Silence: Tragically, the duration is rather short, but that may be all the time you need to make use of this spell. At the same time, it has three Threads, so it's not exactly perfect to use on a target that is running away from you.
Clothing Gone: At three Threads, this isn't a combat spell. However, it is a deeply embarrassing (possibly violating) spell. You're not really going to make friends of the target, but it is a great way to shame them.
Enter and Exit: This will likely see the most use looking for an entrance, rather than an exit, but it's terribly useful and cuts down a lot of the time puttering around, trying to figure out how to get into a location. Information is power and this gives some useful information.
Eye of Truth: The other half to And Then I Woke Up, this helps out with the Disbelief Tests. If you are getting mileage out of And Then I Woke Up, this will help out as well. If not, there probably isn't much to see here.
Flesh Eater: The description isn't entirely clear if anyone other than the target perceives the illusion. Nonetheless, the touch component generally makes this most useful in a social situation rather than combat (along with the Thread requirement). Be aware, the Thread is difficult to Weave.
Flying Carpet: It makes a flying carpet. Which is pretty awesome. There is very little reason to not get this spell one way or another.
Illusion: It creates a relatively simple illusion. Every Illusionist will want this spell.
Noble Manner: A good bonus to social interaction, with a so-so duration (minutes); this is a solid spell worth picking up.
Phantom Fireball: While it has only one Thread, this is similar to, but worse than Suffocation. If you have lots of situations where you can lob area effect ranged attacks to great effectiveness, it may be worth investigating.
Pleasant Visions: A group lock down with a duration in minutes and only one Thread? Yeah, that will do nicely.
Switch: Five Threads is a lot and coming up with a great situation to use this in may be difficult. Honestly, this is more of an NPC escape spell than one PCs are going to use frequently. That being said, the hallmark of a successful Illusionist is lateral thinking and making a round hole fit a square peg.
Wall of Unfire: It will block off a hallway (though anything wider and someone will likely be able to slip around) and has three Threads. The damage is pretty decent and the duration is in minutes, but... it is just so small.