Since Doom came out I've played a lot of games. These days I don't really play at all. I don't know if it's me or the games. I occasionally give something a try, but it doesn't hold my attention.
The ones I spent the most time on were Doom, Doom2, Quake, Quake2, Quake3, UT, UT2, Tribes2, WoW. A few single player games are good, but once I've played them through there's not much more to do.
If I had to pick a favourite it would probably be T2 - best game for real teamwork.