COD 4 - Garbage. The only people who play it are nostalgia-mongering boomers. The campaign is boring and overrated.
COD WAW - Amazing. The first game to feature Zombies and introduce multi-player campaign. The multi-player is slow-paced but fun. The spawns are predictable and the maps easily navigated. It's great but the game was infected by hackers long ago.
COD MW2 - Decent. I never enjoyed this game much but it is the fan-favourite. The campaign is much better than COD 4 and introduces the most notorious villain in COD history Makarov who wants to destroy civilization through warfare for literally no reason; he's just evil, there's no personal gain to be had. It also has this roumd-based Special Ops mode which is somewhat fun but gets boring after two games or so.
COD Black Ops - Transcendental. This game popped my COD cherry and has a special place in my heart. The campaign is great and has an interesting story. Zombies is much better than WAW and features all of the old maps. Multi-player is the best ever made.
COD MW3 - Great. An all-around good game. The campaign is superior to that of its predecessor, the multi-player is better, and Special Ops has been improved upon too.
COD BO2 - Good. Better than most and it follows up on the BO1 story. Zombies is the best part of the game and far surpasses BO1 and WAW.
COD Ghosts - Underrated. The campaign and Zombies are shit but the multi-player itself was fun.
COD AW - Abominable. Everything misses. It was a waste of time making and playing this shallow cash-grab.
The Modern CODs play mostly the same. I stopped playing after BO3 because the story and multi-player were shit and the Zombies was mediocre.