Call of the dead has the hardest starting point. And dealing with George is a much bigger part of the game than the monkeys in acension, or the dogs in other maps. It keeps you on your toes for a larger percentage of the time compared to the others, which may or may not be what you are looking for. I find that co-op is hardest on call of the dead, but solo is in fact pretty easy. 30+ is very achievable if you have a good idea of how the map works.

In my honest opinion, Shangri-la should not even be considered when compared to the other maps. I put some solid hours on that map and was very disappointed. It just felt wrong, it doesn't have the same intense feel as the other maps do. Overall I feel this map will provide the most challenge if you are playing solo. Plus, it's oddly satisfying killing starving children zombies.

Acension is by far the easiest map. The monkeys are pretty easy to exploit during the easier rounds. So you can pick up free perks at ~round 7 if you play your cards right. My personal highest round was achieved playing on this map. The monkeys do start to become a hassle in later rounds, but in co-op they are pretty easy to deal with so long as each player has an adequate weapon for dealing with such circumstances.

Moon was a big step for zombies, it incorporated a lot of new ideas. There isn't just one main nuisance that is very predictable, such as George or the monkeys, rather there is a series of things that could go wrong. This map has some very unique gameplay with the space helmet and the different physics in some areas, so do consider this map if you are looking for a more inclusive zombie experience.