Since there are so many maps, I'll just do a top 5.
1. Ascension: Getting around the Space Monkies, Ascension is my favorite map. The dark, black and white opening, the awesome train areas, the thundergun still in it, I just really liked that map, it was the first map I got to round 30+ on solo.
2. Kino Der Toten: Pure nostalgia. Growing up my dad was the fps player in my home, then my friend introduced me to Black Ops for the wii. He told me about the zombies mode, which scared me at first(I was an innocent middle school kid), then I got to play it and realized that it was incredibly fun. So for christmas I got black ops and xbox live, and literally only player zombies cause' I sucked at multiplayer.
3. Nuketown: Me and my friend are zombie speculators. We were always talking about new map ideas and stories, and Nuketown was always brought up in our "what map should be a zombies map" conversations. When Black Ops 2 was announced and they mentioned Nuketown Zombies, we flipped out. But since we didn't preorder the hardened editions, we didn't get it, so I got a season pass for christmas and got nuketown and enjoyed the heck out of it. I love how its connected with moon and references tranzit. I like how its literally nuketown turned into zombies. Its small, but it has a lot of hidden stuff in it.
4. Origins: Out of all the CODs, its the first to be set during WW1, and its steampunk theme is awesome. The staffs, the story, it tells us more about the O4 and the origin of 115. The mud and panzar soldat annoy me, but I still like its layout.
5. Verrückt: Its a classic horror setting, an abandoned, haunted asylum/hospital, that is overrun with the undead because of a strange substance that doctors used on them. The introduction to perk a colas, Samantha, and the power. The whole story with a creepy teddy bear and a little girl haunting the place literally made Samantha, she was a fan speculated character that became what she was today. It also has more theories we still speculate, Tank being one of the marines, connections with Nacht der Untoten via comms. I really liked this map, and it was really tough with the zombies that ran faster than you.