I can't decide between Fallout, Planscape: Torment or the original Baldur's Gate. The latter two games were based upon popular D&D campaign settings, and both used Bioware's Infinity Engine combined with a hybrid semi-turn based D&D ruleset. All three games were unbelievably rich in both story and character development.
There are characters and dialogue from all of these games that I still remember fondly to this day.
How can anyone forget Minsc and Boo!?!?!?!?! Fucking Brilliant!
Planescape is still one of the most original ideas for an RPG ever. You start off dead and have to remember who you were, and why someone wanted you dead in the first place. You build your character as you learn more and more about things you were able to do when you were alive. You still pick a normal D&D class, but its so subtle you barely notice it. Your class development is based on decisions you make and what methods you choose to use in order to solve your problems.
Plus the game's setting is like nothing else I have ever seen. And there's a wisecracking severed head that follows you around and hits on zombie chicks
I have a weak spot for Baldur's Gate because it was the first PC game I ever played. I was so immediately hooked on that game, I continued dating a girl that I couldn't stand for an extra month just to finish playing it on her PC since I didn't have one at the time.
Fallout took place in a post apocalyptic road warrior-type setting and was one of the first "mature" themed games out there. Not only did it break ground in that respect, but it also had some of the funniest character dialogue of all time. It was rife with inside jokes and sheer irreverence. I loved that game, too.
If I had to choose one to play right now, it'd probably be Baldur's Gate.
And calling Dungeon Siege an RPG is like calling a McDonald's Happy Meal gourmet. That game made Diablo's gameplay look deep. It was a pure hack and slash. Christ, the only thing that made it any different from Diablo was the pack animals and the lack of sexy Blizzard cutscenes.
I had more fun playing Gauntlet in the arcade when I was 10, and it was the same game.