Half-Life 2
simply the best FPS ever made, I can't imagine a FPS PC GAMER thats played HL2 and not like it. 2 words to say why this game rocks, Gravity Gun!
Also no need to say that same goes for CS, TF2 and Portal and so forth.
Halo: Combat Evolved
While I do love Halo 2 and 3 also, 1st game was something else when it was released and just cause of that I hold this game in high regard.
Diablo II
What do I love about D2?
the hacking down of endless hordes?
the endless grinding for set pieces?
the tense feeling of taking a jaunt down a dungeon?
who knows, I just hope D3 is just as good
Star Wars: Knights of the Old Republic
This is the only other RPG besides Fable I can recall playing on the first Xbox but I enjoyed this a whole lot more since I'm a huge Star Wars fan. This also first time coming across a female jedi which obviously become big reason for my love of the game
Guild Wars
Been playing the game for 4 years now so yea I'd have to say a game of the decade for me
Grand Theft Auto
I can exactly say which one to be precise cause I've enjoyed the series as a whole to much to say.
World of Warcraft
Well the game speaks for itself, one of the biggest games of the past years and probably for some time to come. I found the game was pretty good and can see why so many play it but I did find it boring after awhile.
Since I was playing GW before and for much longer, WoW just didn't quite appeal to me after awhile.
Baldur's Gate II: Shadows of Amn
I'm glad to see others mention this also, just quite simply a epic game.
There was quite alot to the game and usually I can be put off by game that has alot to them but I was totally compelled to play it.
Total War Series
I love my strategy games, its my fav genre(maybe tied with FPS) and the Total War series blends both turn-based and real-time strategy perfectly.