I've been gaming many years and I'll list my favorites..

Battlefield 2
Counterstrike Source & Counterstrike Condition Zero
Day of Defeat 1.5
Battlefield 1942
Everquest 2 W/ all expansions - great MMORPG.
WoW - gets boring quick, and sorry but I find it slightly immature because of so many kids.
Guild Wars W/ all expansions - Pretty damn good MMORPG, but low level capacity =/.
Diablo 2 W/ LoD expansion - Classic MMORPG, still a kickass game!
Warcraft 3 - My favorite real time war / strategy game ever!