I'm quite partial to the old Black Isle RPGs.
Baldur's Gate II: Shadows of Amn has a very nice, epic storyline.
Planescape: Torment, which has the best storyline of any game without a doubt, as well as the best dialogue and writing (I'm dead serious when I say this - I have NEVER heard anyone who has beaten the game say its story or writing was anything short of great. Ever.) However, the storyline is not epic - it's about an immortal amnesiac who tries to discover why he's immortal, and why he has no memory.
Although not technically an RPG, but rather an FPSRPG, Deus Ex has a very good storyline and it gets quite epic at the end. The graphics aren't great, but the art direction is excellent.
Mass Effect is always a good choice for epic storyline. The last mission is the best last mission of any game I've played, but that's subjective.
