Probably Die Rise and Origins. Die Rise is the only map that grew on me and got better over time and Origins was the only map this game I felt added new things for the better. Minus the ending of the EE which is the worst thing in any zombie map.
Thats a tough question, initially transit was ace, the bus was great fun, MotD as you guys say had an ace atmosphere, and were it not for the fact I am required to re download loads of crap to play it I would say it's my number one, however as far as I'm concerned they have rendered it unplayable with no real fix to the problem ( freezes). So I say nuke town. It's just about killing Zs, no silly WW allowing players to get to stupid rounds, could of done with a wunder fizz but hey, I still love it