Bioshock 2. The difficulty curve is still awful (insanely hard at the start, gets easier and easier as you make progress), but the atmosphere is great, especially when playing it in a pitch black room.
Edit: Just finished it. The story blows the first game out of the water (pun not intended), and anyone who says otherwise hasn't played past the first few levels yet.
Edit: Just finished it. The story blows the first game out of the water (pun not intended), and anyone who says otherwise hasn't played past the first few levels yet.