I LOVED call of duty World at War. It was also the first call of duty game I really played and my first Xbox 360/Rated 18+ game.
I liked call of duty MW2. Mostly for the pvp, it was pretty cool. The maps weren't as good as they were in world at war, and the warfare was much faster. I felt like WAW was more patience and strategy and MW2 was more fast paced reaction skills.

Black ops was great zombies came back. I disliked some of the weapons. They got rid of the wunderwaffle and that made me angry. They also had cheesier machine guns and seemingly less effective/fun-to-use sniper rifles. Still- the zombies was more modern and was awesome to be expanded upon. Loved the crossbow and ballistic knife. The zombies map on the moon was so mystic.
The multiplayer was also one of the best. Still, I think I liked WaW better back in its day.

I never owned any other call of duties. Although, I liked BO2 zombies when I played at my friends house. Seemed a little too complicated at time