Well to fit your definition of "better" i'd have to say..
Black Ops.
Mw2 was more fun when it first came out than it was for black ops however....black ops is better in more ways than mw2...i mean you can record stuff, its more balanced, no newbies who happen to have killstreaks going pwnin yo ass

and...they ban people from boosting and the like.
But think of it this way the next modern warfare will be nothing short of amazing cause black ops got most of the weak points of call of duty.