I think the games that defined last gen are the ones that had the most influence on devs and most gamers. Games that had aspects that gamers wanted most and that devs had to implement in their games to sell well.
Gears of War (may not be the first with cover mechanics but did it so well other games had to borrow it, also the way the camera shakes when you run... yep a lot of games borrowed that as well)
Uncharted (for it's cinematic approach and character development)
Mass Effect (all around rpg, cover system, deep story, real choices in dialogues, good/evil character devlopment)
Bioshock (incredible art style and ambiance can sell)
Call of Duty (So how many games are trying this formula?)
Elder Scrolls/Fallout (Bethesda knows open worlds and customization, so many rpgs tried to emulate these successful series)
Just Dance (like it or not musical games were a hit last gen, because of JD we got games like Dance Central, Zumba etc ... these games were very present last gen)
Halo (yep devs are still trying to make a Halo killer and still couldn't do it, last gen this franchise went as big as getting an anual release)
God of War (don't believe it when people tell you qte are bad, they like them in GOW. Also Even if only one decent GOW was released last gen, some games tried to replicate the formula, so it was very influential in it's genre)
Wiifit/Wiisports (ohh sure did, get off the couch and start training, yep those "games" were all over the place last gen, because of them both MS and Sony came up with their own versions)
Assassin's Creed (did you say platformer? what game does it better than AC... and those animations)
Final Fantasy 13 (only FF we got last gen... but they made what, 10 games out of it? geez)
Angry Birds (had to put it... really)
