the first black ops hands down had the best. i really enjoyed call of the dead and five and could play those two maps for hours without getting tired. ascension and shang ri la were also fun maps because they were more challenging
Love my some WaW zombies, but I'd say my favorite has to be BO2 just due to the differences between the maps. It's something I don't get tired of as quickly as most of the newer ones such as WW2, BO3, AW, etc.
Probably Kino der Toten, just was a great time back then - playing with school friends and stuff, and just having fun on Zombies.
Going forward, I really enjoyed Spaceland, I just felt there was so much to do, especially solo. Was always fun building all 4 wonder weapons, crafting, finding parts and do the easter eggs on there.
More so recently, IX is awesome. I've put like 60 hours into that map, it's another one with so much to do, and the easter egg is just stupid - so many steps to do, which makes it fun!