Right now I'm playing through the Median XL mod for Diablo II, which essentially makes it ridiculously hard (a one player game can have the difficulty of an 8 player on top of the already added difficulty).
I'm also replaying Dungeon Siege II.
Fable: The Lost Chapters
Warcraft III
Deus Ex
Unreal Tournament 2004.
Both Fable and Deus Ex are probably my favourites, both are great games, and both are rather engrossing.