I didn't buy it, it was a Christmas present. But looking back if I were to have bought it, it would have been for single player and zombies.
Which is funny because I found the singleplayer to be a poor effort compared to the games that came before it. Zombies, while a lot of fun, wasn't as good as I'd hoped it would be. And the multiplayer which I'd thought was going to be complete crap, turned out to be shockingly good.