the furthest i have gotten on zombie mode is round 10. i would say facility is the easiest because you can get around easily and all of the barriers are easy to access. first thing you should do is get as many points you can in rounds 1-3 by getting head shots with the pistol. during round 3 or 4, sometimes five i run to the couch in front of the stairs and remove it. once up stairs i grab something from the misery box a few times trying to get a something that fires about 5 per second if i get lucky. then i go into the room with the red light, it is next to the one with the mystery box. i stand in the corner of the room by the next door you can open and fire at zombies as they come. at the end of the rounds run back to the mystery box and try to get another good gun. and keep repeating till you have enough points to open the next door. make sure you have a full ammo gun, next i stand in the corner by the pshh gun and shoot them as they come and at the end of the rounds find a good gun to refill ammo with and go back to the corner. the pshh gun is the best one in the game. i end up dieing there because there is nowhere else to go.
Pretty impressive i always get overwhelmed at wave 9 , sometimes reached to 11-12 in overlook map. My strategy is always to use the knife the first 3 waves ,then at wave 4 there will be the running zombies , then i use the pistol and shoot them in the head so long when i have no more ammo, then i move further and from there on i buy all kinds of weapon and stuff to try and get further, the most important thing what you gotta do is, to shoot in the head, that will spare you money, or scores lol.
Make throwing knife headshots till round 8, then buy a M14. M14 is the strongest weapon in the zombie game, even at round 27. Just aim for the head, have enough ammo, and maybe a PPSH as a secondary if you run out of M14 ammo.