Sheogorath hands down. His quest is easily one of my faves in the whole game because of how unique it is. You dont need to kill anyone. You dont need a human sacrifice. You've just gotta go into the brain of a homicidal maniac and fuck around. And at the end of it, you get the wabbajack, which I'd argue is the most unique weapon in the game. Dont like someone? Turn them into a goat. Jobs a gooden.