Games are also organized top to bottom within tiers. Also the original MW trilogy as a whole beats out IW, but IW beats out each game individually.
Now I have played World at War before, but never its campaign beyond Semper Fi in co-op so it wouldn't be fair for me to rank it. Though it did leave a hell of an impression on me and I suspect I'd rank WaW highly if I did take the time to actually play through it
Looking forward to reading the discussions in the comments