COD4 is most balanced (Barring Frags). MW2 is most fun to play.

The original Call of duty is the best, to call that game so-so is shameful, It had solid online and the most intense campaign of any shooter - it redefined the way most shooters where made with regards to pacing and action, and of course the inclusion of being part of a large squad.

Call of Duty 4 is the reason as to why Call of Duty franchise is so big and one of the largest ever.

COD4: MW hands down best COD game.

The original for me, it was a small revolution in gameplay when it came out and is still one of the most intense FPS games I've ever played. 2 was ok, 3 was boring.
Modern Warfare is solid but way too cheesy for my tastes as far as the "story" goes. World at War is by far the worst in the series and is the only installment (except MW2) that I haven't finished.
MW2 was a massive letdown for me, its more over the top than the first one but that isn't always a good thing and it resorts to some of the cheap gameplay elements from WaW that I hate intensely (infinite enemies unless you advance etc, makes the whole thing incredibly unrealistic).

It's World at War for me. I love history and World War II is the largest war ever, so far.

COD 2. Nuff said

Cod 4 for me. I was so blown away the first time I played it and very few games have given me that feeling. WaW and MW2 are tied for second place though.