I have to disagree about Call of the Dead, I actually find it quite easy even with George in the way. Granted, he's a pain in the ass if you accidentally hit him, but it's not necessarily game over unless you're in a bad spot. I choose a good place to train zombies on that map and George ALWAYS comes to it, so I've learnt to train whilst avoiding him. The only times he's been hit was when other players would shoot the scavenger at my train from a distance - drives me nuts!! Focus on your own damn zombies especially when you can see George is with me!!
Top 3 hardest maps for me:
1. Shangri-La
2. Nacht der Untoten
3. Verruckt
(either hard to train or lack of perks on these maps)
..closely behind is Bus Depot due to lack of perks, but I find it's do-able with 2 players taking each side of the map. Got over 20 rounds this way.
Also surprised to see people saying Five and Moon - easy!!