Everyone knew this was coming
I can only call that my favorite PC game though, I've always been a console gamer at heart so most of the heavy hitters (MGS, FFVII, Goldeneye, Zelda, etc.) have been mentioned - oh, but big +1 to Half Life 2 (and even bigger +1 to HL2 > HL1, thanks to the introduction of *gasp* characters with names and some semblance of a story), and also, I'll throw out some of my unmentioned and/or lesser-known personal favorites, in no particular order:
(Most diverse but well-developed character cast in any game I've ever played, bar none)
(Yes, seriously, the plot is braindead and somewhat jingoistic, but the set pieces are just so mind-blowing, and the whole presentation and production so slick, this is modern shooter perfection as far as I'm concerned - the video game equivalent of Nickelback's "All the Right Reasons", perhaps
And Soap + Price are definitely up there on my "most badass video game duos" list)
Definitely intentional that the majority of those are the first sequels in their respective trilogies, as my experience is most of the time that's the peak for a series