X-COM, X-COM. I can't recommend it highly enough. I think they even have it on Steam. I know I got it with the 2K pack, or one of the other multitudes I bought.
Also Heroes of might and magic 3. That sucks up hours and hours of time. I also highly recommend Alien vs Predator 2, much better than the one that just came out. I played a lot of Warcraft 2, but that's one game that had games top it. I'd also suggest
Edit: Looking at my CDs here some more. Thief 2, definitely Thief 2.
Edit 2: Duke Nukem 3D, if only because of the shrink and freeze ray, and the strippers.