Has to be 3D for me, but it has to be done correctly. It's easier to make a 2D game look great with the right artist (like in the larian games) but when it comes to immersion into a game world I prefer 3D, but only recent games, because only now are the textures and lighting getting to a realistic stage.

3D is harder to get right but they've really come along in the last couple of years - morrowind was good (but too slow) but when I play something like thief deadly shadows with its great lighting and physics (apart from the hilarious rag doll people) I get pulled into the game far more than a 2d engine could ever hope for. Just picking up stuff and throwing it around is fun.