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.