Okay I will attempt to rank the zombie maps from my favorite to least favorite with some reasoning involved.
1. Der Riese - Just a really fun map. Has a wonderful feel to it. Not too simple, but not too complicated. All around fun.
2. Ascension - Wide open space and more great fun to be had. Monkey rounds are dumb, but whatever. Doesn't hinder this great map too much.
3. Moon - Another awesome zombie map. Wide areas and unique with it's zero gravity etc. I'm just not a fan of how complicated it got. the whole excavator thing is so silly with the hacker and such. Just the idea that you can be stuck on one side of the map with literally no way to escape is a bit much. Overall an awesome map.
4. Nach der Untoten - Smallest and probably hardest zombie map. But there's something great about how simple it is. You don't have to worry about point conservation and when you can buy Jug and other things. Just straightforward knocking out rounds.
5. Shangri La - This map's ****ing hard. but I really like it. Nice colors, but can be annoying at times.
5. Kino Der Toten - My first zombie map I ever played. And it was a great beginner's map. Big full circles and a straightforward layout. A great way to introduce someone to zombies. A bit bland now that I think about it, but fun nonetheless.
6. Call of the Dead - Probably one of the more controversial maps out there. I seem to like it more than most. the real turn off is George Romero. The narrow spaces don't bother me. Overall i actually found it to be a fairly easy map.
7. Verruckt/Shi no Numa - Honestly have not played these much at all but neither impressed me too much. I'd probably give Shi no Numa a slight edge on verruckt. verruckt I admit is pretty damn hard.
8. Five - Five blows.
9. Dead Ops Arcade - LOL
I think that's all of them. I love all the zombie maps to be honest. Except DOA. I even like five a little bit.