CoD4: Modern Warfare
My attitude to this game was the same as Activision's, ie, not WWII = will be shit.
I picked it up just after launch for a fiver by a complete miracle. I was going out to two games that I actually wanted: The Simpsons Game and Ratchet and Clank, when my dad chucked a load of coupons at me that he was about to bin, so I decided to pick up a third game, was CoD4.
Didn't even play it for at least a month, put it in, fell in love, and was by-far the most played game of the generation for me... ended up rebuying it for both PC and 360.