For 360: gears of war. Bloody shooter w bro dude crowd and set tone for tons of other similar games. Also sold a lot of 360s.
For ps3: uncharted. 3rd person action adventure that showcased the power of the ps3 and set tone for similar games. Brought naughty dog into the public eye and sold a lot of ps3s.
My overall pick for best last gen game... last of us. Why? I'll remember it in years to come. Can't say that about many games. Even ones I love.
I'd second this if for no other reason gears was the first title that screamed next gen to me and I played the ever living s*** out of that game. Uncharted was easily one of the biggest franchises for sony last gen and the last of us was f****** amazing. I can't think of many games without Zelda or Mario in the title that I've beaten as many times as I have that game.
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Yeah gotta agree the only answer is COD4. Not just for the game play (which has left an indelible mark on shooters and games in general), but for popularizing YouTube commentaries. It was such a perfect fit for the medium and created a subculture which didn't exist (to that extent) before.
Minecraft, Candy Crush and Wii Sports probably come in equal second for different reasons. I don't really rate Angry Birds in this respect as I think its cultural impact is far greater than its impact on gaming.