I'd have to say Oblivion. It has its downsides, but it also has great sentimental value for me as the first game to really get me into RPGs. The scope of the game astounded me when I first played it, and I put so much time into it that I practically know every pixel like the back of my hand.
Okay, so I don't, really. Sue me.
It's still the best I've played.