For me the game of the generation (and greatest game of all time) is Team Fortress 2. When it first came out it in 2007 had 3 game modes, 6 maps, and a single set of weapons for the 9 classes. Now in 2013 we have several more game modes, including a a coop Hoard mode. The number of official maps have expanded to more then 50, all of which were free for all players (no $15 map packs here). Each of the 9 classes has a dozen or more new weapons, some of which significantly change the way the class plays. The game still receives several major updates every year which includes new weapons and maps.
The gameplay is just amazing. Each of the 9 classes is very different. The scout has very little health, but is extremely fast and can jump multiple times. The medic on the other hand very rarely engages in direct combat, but is often one of the most valuable and one of the highest scoring players on the team.
Some of my other favorite games released this generation include Left 4 Dead 2, Portal 1/2, Super Mario Galaxy 1/2, Batman Arkham Asylum/City, Alan Wake, Dragon Age: Origins, Skyrim, Fallout 3/New Vegas, Deus Ex: Human Revolution and Hotel Dusk:Room 215/The Last Window.