I'd have to say BG2 by a hair over Icewind Dale just because of the sheer depth and size of the story. For me, I never quite connected with my party in BG2 as I did in Icewind Dale because, even when you customize the whole party, you're playing an established character. Icewind Dale's characters are entirely your own, it felt a little more like the old school D&D sessions I used to play with pencils and paper. Maybe the Icewind Dale expansions will add some real depth to the gameplay. Diablo never really appealed to me, it's an action game really, not so much an RPG (in my opinion). Of course, you wouldn't go wrong playing any of Black Isle's fine RPG's.