The space monkeys are fun opponents. They make Ascension a unique map to play. It is however not my favorite map. My favorite map would have to either be Der Riese, Shi No Numa, or Kino Der Toten. It's hard to choose a single one. I really like Der Riese on Black Ops 1 because you have a mix of WWII and Cold-War era weapons. Also, it's the only map you can Pack-a-punch WWII weapons, namely the FG-42, STG-44, and Thompson M1A1. It's also the only map to feature the pack-a-punched Wunderwaffe, and Hellhounds mixed with zombies. So it is also a unique map when you think about it.
Shi No Numa is an interesting map as well, not only for the setting but also for the fact that it's the first map to not feature Nazi Zombies, but Imperial Japanese Zombies. Furthermore, it's the first map to feature 'dog rounds' and the Wunderwaffe. Anyway, I enjoy this map because it's a challenge. Perks are randomly set, and the swamp makes running a lot tougher, even frustrating. It's a personal favorite of mine though because it features the BAR. Verruckt has the BAR as well, but it is a small and tedious map in comparison. There is much more room to run on Shi No Numa.
Kino Der Toten is also a favorite of mine because of it's simplicity. Anyone can play that map and have a good time. It has Hellhounds...it has the MP40... it has the Thundergun. Pack-a-Punching is easy. Reaching mid-tier rounds is easy. What I find interesting about this map is that while it's yielding in many respects it is also actually quite challenging in the later rounds, if you don't know what you're doing. Most people try to run around the map like mad men, which can and does work, but you often get people who tend to run the opposite way for whatever reason and get everyone trapped. If you have one person running in the alley, one person running in the lobby, one person running right below the stage, and one person running on the stage, I think you could probably go pretty far.
Anyway, those are my favorite maps.
MoTD (So far) It's such a nice and big map, the warden isn't easy but it isn't hard either, making it a good boss. The pack a punch is fairly hard to get to, but it's very rewarding. The wall guns are perfect, electric cherry is a fun perk to use, the gondala's are the ultimate trolling devices >:D, the golden spork and hell's redeemer (I hope the redeemer makes a return in the next dlc's) are very rewarding side-quests, and finally, the blundergat. That thing is a beast. I pack a punched it ( without acid upgrade) and I went to around round 35 with my friend.
MoTD is just perfect.
I'm sure people will disagree with me, but this is my opinion.