for online experience, black ops. maps are better because they were not also designed for survival mode. there is more diversity in guns, and wager matches are amazing. the money concept is nice as well.
i played every cod from 1 to mw3 and i think that 1,2 mw1 and united offensive (the expansion for 1) are not only the best but the only really playable keep in mind i played all on ps3 and pc so idk about xbox
I'm a big CoD fan, backlash be damned, and the thing that separates Black Ops from all the other titles in the series is the story. I think everything about it -- the voice acting, the structure, the narrative frame, the characters -- is fantastic.
But that's just me. I'm big on stories and narratives in single player experiences. I'll concede that MW3's MP offerings are richer, though I myself prefer the Black Ops maps and modes.