I have assassins creed 2 and L4d2 but im constantly playing AC2 its so much fun and you get so into the story, i play it more than Modern warfare 2
Well If you only want to play a story mode and not have much replay value then Assassins Creed 2. If you want to play online and your going to play one of these games a lot then Left 4 Dead 2. I have both games and I only played Assassins Creed 2 for the first 3 days I had it. It gets boring once you beat it. I'd recommend Left 4 dead 2.