Far Cry is the best game I've ever played, single and multiplayer
It's better than Half-Life 2.
Other games I've loved and would recommend:
Max Payne 2 (too short though)
Splinter Cell: Pandora Tomorrow
NFS: Porsche Unleashed - awesome game for like $10
FIFA 2004, 2002
Civ 2 (easier than Civ3), Civ 3
Medal of Honor: AA
Half-Life of course
I've played both Halo's and I didn't really like either of them.