I don't really want to say this, but I'm not sure you can get away with a top 3 defining games list and not have World of Warcraft on it. I lost count of how many records it broke this year (which were previously held by Blizzard anyways).
I was involved in the alpha and beta testing, and they aced the entire development process. Alpha was fun. I was trying to hunt for bugs - not have fun - but it still got to me. Beta testing was basically the real game with some lag. There was hardly any serious hang-ups. They set a deadline and met it as well. I don't think I even have to mention the staggering 11.5 million accounts (but I will).
All in all, Blizzard executed the release of Wrath of the Lich King perfectly (or near-perfect).