Here's how I see it:
COD2: A bit challenging, but alright
COD3: Impossible. I still haven't finished it
COD4: Easy. Only moderately challenging part was the "Mile High Club" achievement
COD: WAW: An unlimited barrage of grenades. I'm pretty sure that there weren't even that much thrown in WWII
COD: MW2: Easy campaign. The only things difficult were the Spec Ops "Wardriving" and "Wetwork"
COD BO: Extremely easy campaign. Challenging parts: zombie achievements on maps past Ascension.