I approve of this thread.
Deus Ex, on realistic difficulty. On missile silo level, can't anymore remember how many times I have already played through this game. Doom, Doom II and Final Doom - I play these daily, although I rarely play the official maps anymore. Fallout. Duke Nukem Forever. And UFO: Enemy Unknown, of course.
Looking forward to these games: Deus Ex Human Revolution, Hard Reset, XCOM, Thief 4, Doom 4 and Rage.