Final Fantasy XI, Dervishes in Guild Wars, Masterminds in City of Heroes, Wardens in Lord of the Rings Online.
While some of the aforementioned classes might have vague counterparts elsewhere, there's nothing else that captures the same feel, and that makes them more memorable in the long run. So what's your favorite unique class (or set of abilities that are loosely shaped like a class) in a game? Are they your favorite class to play, or just the one that felt the most distinct? Do you still play the game in question for that class, or have you since moved on, hoping to one day recapture the fun in a new setting?