So far, I've DMed D&D 3.5 (Vanilla and Iron Kingdoms), D&D 4.0 (Vanilla), New World of Darkness (Vampire, Werewolf, Mage, Changeling and Hunter), Warhammer Roleplay, Paranoia and a homebrew Iron Kingdoms.
Iron Kingdoms is still the best setting I've ever ran, but sadly noone else has the balls to run a game of it so I can play because I can still quote facts and figures about it off the top of my head
I've loved every second of DMing I've ever done, from the joky games where everyone gets their missions from a shifty looking halfling that insists on meeting them in a Tavern at ten in the morning to deadly serious World of Emo-crap (which I ran precisely ONCE before I got everyone to roll up werewolves and ran a game based on hunting spirit monsters in the Gauntlet
)
EDIT - Also, Inquisitor and another homebrew based on Vor : The Maelstrom.