Call of Duty/United Offensive was horrid purely because I had to stop and restart the missions over and over because I did something wrong. Not something like killing my teammates. Once, I picked up a rocket launcher too early. Another time I went to a side instead of running through a bunch of people shooting at me so I wouldn't die. I don't know who made that one, but it was bad.
Call of Duty 2, 4, and 5 (World at War) were all good in my opinion. As for campaigns though, 4 was a let down, 2 was ok, but I have to say half of 5 was the best. The American side was boring, but the Russian side was amazing. I never played 3, heard it was bad from someone, but was never interested in it. MW2 I refuse to buy for certain reasons as least on PC. Might rent it for PS3 just to play the campaign.
I think they are about equal (comparing 4 and 5), but I like 5 a lot more.