Hmmm.
1.)Medieval:Total War was pretty good, though a bit annoying. Rome:Total War will probably be better.
2.)Far Cry. I haven't played this properly, but hell, it looks amazing, the A.I. is pretty good too.
3.)Half Life. Bit dated now of course, but an authentic, innovative approach to the genre.
4.) The Baldur's Gate games. NwN is let down by a poor single player campaign.
5.)Age of Conquerors. Age of Mythology is a titillating game, but it never felt as addictive or potent as it's predecessor.
Halo was quite fun online too...