i think that it depends on which you played first, me having bought WAW first, like those maps better because i spent hours trying to find their secrets and strategies (there are quite a few on der riese btw, my favorite map) also world at war has more of the story centered somewhere in it's maps. so my vote goes to WAW
Black Ops because of new perks and possibility of more maps coming out. Kino der Toten is my favourite map. Acension is pretty cool also because it had tons of new stuff in it.