It is embarassing how few of these I actually played. Too little time to play games, although its my only real hobby.
Anyway, of the ones I played, I think I would pick the Witcher here. I though the ending, including the dream-like ascent to the temple and the revelation at the end were particularly memorable, and a fitting end to a cool game.
Fallout was cool, but as I said before, I came to FO late, and it didn't wow me as much as many others who played it at the time it came out.
The ending of Gothic (as opposed to the rest of the game) I remember actually as a bit of a dissapointment. And yes I know, I really have to play Ps:T ... one of these days.