Ok, Captain Obvious here, but Minor Image and Major Image are musts, IMO. Of course you need to make sure of how your CK is adjudicating illusions. These spells are limited in use only by your imagination, and are of course the bread and butter of the Illusionist.
Light, Detect Illusion, Message are great 0-level spells.
Beyond that for using on friends...
Wards Temporary Strength - can make someone get an extra +1 perhaps, huge bonus (+5% chance to hit, nothing to sneaze at) and can stack to get someone up to str 20.
Faeries Glammer - Imagine making your tank look like the Illusionist and have him boldy call out a challenge to the big baddy
Invisibility - Let that thief get that critical sneak attack
Of course Dragon Armor/Blur
For perhaps saving posteriors when time to run...
Ghost Sound, Illusionary Hounds, Obscuring Mist, Fog Cloud, Misdirection, Rope Trick, and Illusionsary Hounds are invaluable.
There are direct damage dealing spells, I tend to avoid those and leave that to our mage and warriors, but they are effective if you want to use those. I just feel that with a good CK and a good imagination pure illusions are more useful, and quite possibly much more deadly (all things being considered), than just damage dealing spells. Some combat spells like color spray and hypnotic pattern are great pure combat type illusionist spells that fit the fluff.
Same goes for healing... if your group embraces illusionists as healers, then a must, our group doesn't like it so much. That doesn't mean you can't take them unless outright banned, but think of the fluff of the character not pure min/max.