Dang. my list would go a little something like this:
Buried- too easy, get set up at round one and then camp jug for the entirety of the game.
Ascension- has some of the easiest training spots in the game, the easiest wonder weapon to use, and traps that kill full hoards at any round.
Kino Der Toten- same as ascension with a better camping spot than any other map (fire pit).
Der Riese- has a ton of spots that are easy enough to train, and traps make high rounds possible, as well as max ammo from dogs at higher rounds.
Shi no Numa- Shi no Numa's swamp has always intimidated players, however there is nothing to fear. The training spot in front of comm room and flogger make for easy rounds, not to mention the wunderwaffe.
Tranzit- has unlimited ammo wonder weapon along with massive training spot in front of the farm. Also persistent upgrades allow for higher health, retaining of perks, etc.
Mob of the Dead- This map's cafeteria has the best of everything: easy access to trap, zombie shield and wall gun for points to feed the trap.
Die Rise- Okay, the Budha room would be an easy place to train, but the zombie AI and walkers on this map are what makes it harder than what lies above. Sliquifier was way nicer pre-patch.
Farm- Barn spawn control is the one reason this gets ranked over town. One person at window, one person at barn opening = round 30+.
Town- has a few training spots, but overall the lava makes it annoying. Easy to jump over when you teach yourself. Raygun Mark II is a beaut on this map.
Nuketown Zombies- No lava makes this town easier than the first... until you remember the perks come randomly.
Origins- The easter egg is a hard task to complete even for the best of players on solo. Mainly due to one step, but still. Panzer in early rounds is no joke, take him out quick.
Moon- Moon has some great areas to train and great weapons to use, as well as a max ammo farmer. However, its ease is reduced by- excavators, no man's land, and the ever-annoying cosmonaut.
Shangri La- The wonderweapon and map is amazing. There are unfortunately no usable traps, so hitting box mid-round for that baby gun will prove difficult indeed. The boss zombies? Shrieker can be defeated easily enough, but the Napalm zombie will prove a little bit more difficult. Training either includes whole map trains or cutbacks for days.
Call of the Dead- A full map without any instakill weapons (for solo play) or traps? Hitting the box mid-round? The only thing you have to save yourself with is the crossbow and George's gift.
Verruckt: No high round killing weapons and traps that are debatebly the hardest to run in zombies, this map is no easy task.
Five: Even harder traps to run than Verruckt. Good luck. (Still one of my favorite maps to this day)
Nacht der Untoten: no jug. Need I say more? Thundergun? Oh, right.
Bus Depot: No perks? No thundergun? Wait, there's also lava? Only Jesus could survive on this map.