Fallout 3, hands down. I like it primarily because it breaks out of the Tolkien-esque mold that RPGs have been stuck in for three decades, which is not to say that those RPGs were not good games. Morrowind was spectacular.
Yes, I know that the S.T.A.L.K.E.R games were not RPGs in the traditional sense of the word, but the experience is very akin to that of an RPG, especially with regards to the quests. Clear Sky came the closest to becoming an RPG with the integration of the factions system (which was downright wonky at times). Then again, maybe I just feel the need to express my love of the S.T.A.L.K.E.R series by pointing out that it is really the most even hybrid of FPS and RPG ever made (apologies, Deus Ex).