With so many games to chose from old and new it's best to categorize my favorite games on PC.
FPS - Half Life series, Battlefield series, Crysis and Crysis Warhead, Far Cry 3, Dishonored, Fallout 3.
RTS - C&C series bar 4, Sins of a Solar Empire, Homeworld, Starcraft 2
3rd person - GTA series, Fable, Assassins Creed series.
MMO - World of Warcraft, Star Trek Online, Lord of the Rings Online, Planetside 2.
RPG - Elder Scrolls series, The Witcher series, Dragon age.