Mines has to be Ascension not sure if thats how u spell it, its my fav because its an open wide map and the monkeys are not that hard to kill but it can be a challenge sometimes. Getting to packapunch can be a bit of a challenge sometimes but thats what i like about it.
I think that in the beginning rounds, monkeys are very easy. They get progressively harder later, though. The part that makes them hard is probably their small stature, so they always seem to get to the perks easily and unexpectedly. I think that it is also harder to see them jumping in later rounds. When they jump, you should jump too to avoid being knocked out for a bit. Anyways, they are fairly annoying, but they are part of what makes ascension fun.
The hard part about getting to pack-a-punch would probably be the need to open so many doors to get to the place where you launch the rocket. It seems as if once there, you don't even have enough money to pack-a-punch anymore! I guess you should only go there once you rack up an extraordinarily large amount of money. The pack-a-punch room is a great place to run circles, though, so I think it is worth it despite being able to pack-a-punch or not.
Ascension has many places to run around in. For example, the stage with the launch pad is a great place to go after you get out of the centrifuge room. Afterwards, you can run circles around the entire complex just below... there are places you can jump down if it gets too tough, and there are many useful traps (unlike Kino

). I really like the fire wall trap that you can shoot through without risk of being hit. The traps in Kino are really not worth it. Then, of course, is the pack-a-punch room, which is a vital place if you want to get to the higher rounds. Because of these separate places, it is very easy to fight independently and avoid conflicting zombie lines, which happens a lot to me in Kino. It is harder if someone goes down, and you have to revive them, but it should be pretty simple to get around the zombies. Pack-a-punched ballistic knife would be really helpful in that case.