Rogue - here's why:
1. Warriors are great fighters, but that's about it. Trap? Locked iron door? Out of luck.
2. Mages are great in terms of sheer spectacle, but growing up with AD&D 1E I can't help but think of them as walking corpses prior to 6th level (when they get 3rd level spells - i.e., fireball).
3. Rogues are the utility infielders of role-playing. They can kind-of do anything at least sort-of.