For the PS3 I'd say that the best games are Uncharted 2 and Metal Gear Solid 4. For the 360...the only 360 exclusive I played was Halo 3 and ODST.. That I really liked.
Halo 3 is probably the most popular game on the xbox 360. Many people have many cons for this game, but it doesn't stop them from buying it. It is (in my opinion) the most popular video game of all time. Yes, Call of Duty and all the other games are awesome and better than Halo 3 but think about 1 person who has an xbox but doesn't have Halo. Not a lot of people. Feel free to argue with me, but this is just my opinion. As for the Play Station 3, I am unsure because I don't have one.