It depends on what you want from a FPS. This year, I've spent more time playing Nazi Zombie mode in Call of Duty: World at War than any other game. Not because it's technically superior to anything else - but because it was so addicting and satisfying that I didn't *need* anything else. What started out as merely a "prototype", Nazi Zombie mode has become something really amazing and addictive. Most people who  take jabs at World at War only do so because they judge World at War solely on how its multiplayer stacks up against COD4's multiplayer. I will admit that, if you don't care for old guns or the WW2 theme, then you won't find much mileage out of World at War's multi. But if you love online cooperative zombie mode, you will love Nazi Zombies mode and all of its DLC content. Left 4 Dead straight up sucks compared to it in my honest opinion - I find the old Source engine to be far less immersive than many newer game engines.  Also you can play the campaign cooperatively (unlike in COD4). Don't get the wrong idea - I think COD4 is one of the greatest games ever made. But there's plenty of "gaming needs" it doesn't facilitate that WaW does. I get sick of people not being honest about this. I think I just hate humans. That's a whole other thread though. Hope this helps.

WAW should be at the very bottom of your list.