Grease, it's powerful and funny.
Glitterdust, it's just really powerful.
Obscuring Mist, because for the low cost of a 25 gp scroll you get to demonstrate how completely worthless the entire Rogue class is at combat.

Wall of Stone, for the shapeability.
Dim Door, because being grappled sucks
Dimension Steps, for instant tactical repositioning
Benign Transposition, the poor man's Dim Door
Orb spells, though I wish they were Evocation instead of just making the entire Evocation school cry
Solid Fog, instant army shut down
Evard's Black Tentacles, why settle for just beating enemies when you can demean them as well?
Heal...no explanation needed
Close Wounds, saves someone from dying.
Maze, because I'm in favor of punishing people for stupidity
Any summon spell that lets you get a Lillend, Succubus, etc..., with Extend Spell
I guess what I'm trying to say is, Conjuration is awesome! And I'm sure I missed a whole ton of great/fun spells, too.