Best game ever? Do you get to kill zombies in it? You get to kill zombies in Left 4 Dead (also has coop campaign mode) and also in Call of Duty: World at War, you also get to kill zombies. In Resident Evil 5 you also get to kill zombies.
So, there you have it. Because you can't kill zombies in KZ2, it is suspect as to the best game ever.
I will say it has the best GRENADES ever though. That smoke trail at that shows the path the grenade is thrown, is AWESOME.