Three of my favourite PC games (not ancient, mind you, but probably "old" by industry standards):
Outcast (1999), Beyond Good and Evil (2003) and Black and White (2001).
The first two were adventure games that had addictive gameplay and gorgeous graphics for their time while "Black and White" is a god game where you have waaay too much power over your subjects (I distinctly remember going 'Old Testament' on their asses when I got fed up with being a good/kind diety).
Currently, the only non 'state of the art' game I'm seeking out is Grim Fandango (1998). I keep getting the files via torrent, but it refuses to run on my system
