COD: Modern Warfare 1 and 2 VS World at war (Xbox 360)

I've been wanting to buy a COD game for a while, but havent been able to see which is best (I've heard rumours that modern warfare 2 was overrated) . Which is best online? Is Nazi Zombies that good? Which has the better campaign mode?

I havent played WAW, but Zombies just seems like survival to me,...Although i would normally pick COD4- Mordern Warfare over MW2 any day, campaigns on both are relatively fun, though I am pretty sure the original is more challenging and feels more rewarding, unfortunately the multiplayer is at a general low, and isnt quite as flashy as MW2, I would suggest renting them before making a decision. (and keep in mind Black Ops is coming out soon)