Fallout 3 is definitely on my list to replay some time, not because there's a lot of choice and opportunity - even though I'm pretty sure I haven't seen at least 20% of the game world - but especially because of all the mods that I haven't tried out yet. As a FPS, Fallout 3 was pretty weak, but there's a lot of mods that can fix that, to the point of having a good chance to get your character stuck in a situation where he has no chance of getting out alive. That's post-apocalyptic survival done right.
Bioshock, though... It'll be a long while before I play that again. Don't get me wrong, the game was brilliant, but much of the quality came from the story and the ambience. When you've already been through both endings, the experience is just much less satisfying for me. It's suddenly painfully obvious how often Atlas says 'Would you kindly'.
I definitely have to replay Mass Effect too, at least once before ME2 comes out. There are all those medals I still need to get, and you need at least 3 playthroughs for that. But the thing is, I'm a whore for 100% game completion so I just can't let most of those side-quests go unfinished. That makes the following playthroughs kind of a chore.
And then there are those games I just have to replay for pure nostalgia. I grew up with playing Quake 2, Unreal and Half-Life, so those are a must to replay once in a few years.