For me it's a tie between Morrowind and Fallout 3. Morrowind is still one of my favourite games, and introduced me to western RPGs in the first place, but Fallout 3 basically took the things I liked about Morrowind, Oblivion, and Fallout 1 and left out the things I didn't (for the most part, and for the rest there's mods). Close 3rd is Mass Effect 2.