1. Color Spray - AoE save or die at first level? Levels well for a few levels, too? Sign me up!
2. Invisibility - the game changer.
3. Major Image - at this level this is the best you can get for versatility. Not as good as Silent Image for its level, but still awesome. Levels extremely well.
4. Shadow Conjuration - assuming you can fail the save voluntarily... otherwise, Greater Mirror Image for sheer DM frustrating power.
5. Shadow Evocation - you can now mimic the one school your illusionist banned fo sho.
6. Shadow Walk - yes, more shadow, give me more shadow spells! This is great not only for travel, but also for escaping a losing battle, or bringing an opponent with you, re-materialize, dismiss Shadow Walk.
7. Project Image - I wish I ever got a chance of using this spell, it sounds like two tons of fun in a one-ton barrel.
8. Greater Shadow Evocation - also known as the Illusionist's Contingency/Wall of Force.
9. Shades - for lack of better options. Can mimic the whole Conjuration school, not only certain subschools, so this is strictly a good spell. Doesn't hold a candle to other school's capstones, though.
EDIT: Huh. Just noted these are ALL in the PHB. So much for unbalancing power creep by way of too many supplements.