From a mechanic viewpoint: Transmutation. So much useful stuff! (Fly for Pete's sake!).
But from a flavorfluff viewpoint, I like conjuration so much that I had a character whose magic always took the form of a "summoned" creature. Magic missle appeared as glittering blue elven archers, and Leomund's Tiny Hut was constructed by an army of quickhanded, magic gnomes.
I love Enchantment and Telepathy for psions, but I gotta say I spend most of my spell slots on transmutation spells (with conjuration a close runner up).