1. Nacht Der Untoten - Especially from World at War, since there are no perks (other than Mule Kick in Black Ops) and no Pack-a-Punch. It's also pretty difficult to train in, unless you use the whole map but even then it's difficult without Jugganog.
2. Verrukt - Only from World at War, Black Ops version is a LOT easier. Mostly because the only Wonder Weapon available is the Ray Gun (which I hate anyway) and the zombies' maximum speed is as fast as though you were on the Bus from Tranzit or the Tank from Origins, also they get to this speed at round 6! My final points apply to Black Ops as well; the only way to train is throughout the entire map, and finally there's no Pack-a-Punch.
3. 'Five' - I don't know if it's just me, but I found training on this map very difficult. The only place I felt I could train safely was on the 2nd floor. Since the top floor was very cramped and the bottom floor had those annoying gas zombies everywhere.
4. Shangri La - Again, a very tight map with the only viable option for training being most of the map. The napalm zombie was also incredibly irritating; he was very slow so he wouldn't ever be part of the train you were running, instead he'd be lying in wait to blow up on you as you go round! Also power ups were almost obsolete if you didn't get them within a few seconds because the monkeys would run off with them!
5. Nuketown - Although it is very easy to train on, how well you do up until about round 20 mostly counts on luck since you have to wait for the perks and Pack-a-Punch to fall. Imagine having to go till round 20 without Jugganog! I usually end up restarting my attempt if it's not the first or second perk to drop (discounting Quick Revive if going solo).
I didn't include any of the survival maps from Green Run as I don't think they really count as proper zombies maps, but if I had I'm pretty sure I would have included Bus Depot and/or Farm since they're pretty difficult due to what they're missing compared to other maps. In my eyes I see Bus Depot as a kind of tribute to Natch der Untoten since it's very similar in how much it has to offer, but it's much easier to train on despite the lava.