Red Dead Redemption
It was simply just a lot of fun. Loved the setting, loved the characters, loved the story. I platinumed it too, so clearly I liked it
A close second is black ops 1. I played the shit out of the MP
Platinums: Red Dead Redemption, Killzone 2, LittleBigPlanet, Terminator Salvation, Uncharted 1, inFamous Second Son, Rocket League