Oh, such a great year.
Half-Life 2, GTA: San Andreas, Fable, KOTOR II, Tom Clancy's Splinter Cell: Pandora Tomorrow, Far Cry, Rome: Total War
The two I like the most most are The Chronicles of Riddick: Escape from Butcher Bay - not only it was fantastic game in itself, but to this day probably still the best "game based on movie IP".
Other one is favourite of mine from that year, and one of mine all time favourites, Vampire: the Masquerade - Bloodlines, the very definition of flawed masterpiece. RPG with lot of choices that really alter how you play the game, with great system powering it and soaking in World of Darkness atmosphere. Swan song of Troika Games, but what a song it was.
But not matter what is my favourite, this is the year that brought to life juggernaut that is World of Warcraft, so my vote goes there.