in no particular order and since I feel like a Rebel i'm giving you 6.
1) Left 4 Dead. Reason? It's the zombie apocalypse. Bring friends. 'nuff said.
2) Supreme Commander. Reason? Slap your self in the face and think about it.
3) Battle Field 2. Reason? Thriving community even after a long period of time, uses veichles huge maps, essentially got all of the basics down and then some. Oh and when it came out it looked GOOD.
4)Half Life 2. Reason. Admittedly gameplay wise it isn't a crap load different from the first, but I loved this because A) it had one of the first in-game physics engines and used WELL. B) Graphics looked nice and IMO still look fantastic *note* I'm not a graphics whore IMO CoD 2 still looks nice. */note*
5) Portal. Reason? http://www.youtube.com/watch?v=b6Q_koTHB54 'nuff said.
6) (six baby, oh yeah I'm a rebel. =P )Unreal Tournament 2004. Reason? this game is the reason I'm a gamer, sure I played CoD 1 and kicked my dads ass even when he was playing it for months. but I never thought much about gaming, atleast not until UT4, I was 10 when it came out and I swear to god this sold me. I loved the graphics, I loved the over the top attitude, the fucking WEAPONS MAN I was convulsing at the awsomeness of it. it sold me and it keeps a spot in my heart. that being said IMO UT3 was a fuck up with no survival mode. (it's my opinion so bitching about it changes nothing nor do I care that you think UT3 was awsome.)
-------------------------------------------------------------------------------------------
---------------------------------------Close Contenders!-----------------------------------
S.T.A.L.K.E.R. Shadow of Chernobyl. It was good but the story never got interesting.
Fallout 3 Damn good game, but frankly I must be hella impatient because it seemed alot like a walking sim.
Team fortress 2 Wasn't enough room on the list, and considering I now reside in the country with wireless internet, ping becomes my enemy.