1) Der Reise, great map for good team work and solo runs, and the best PaP weapons (Barracuda, Longinus,Reaper,) and regular box weapons like fg42, Wunderwaffe and Flame Thrower ( and not over powered) and my personal fav sniper rifle PTRS
2) Shino numa/Kino Der Toten, Both great maps with area's big enough for whole teams to set up or for solo/team train runs round the whole map (CoTD is good for this as well) Can't decide which one is better.
3) Call of the Dead/Moon, Big maps lots to explore (Killing George is fun), Both are great for team work and solo runs and can keep solo players busy doing easter egg (although moon you can't complete it solo)
Maps worthy of mention- Shangri-La, Great map tight spaces 4 people looking to get all perks each could find themselves playing one game that lasts a day, multiple enemy types.
Five- Good map, buildable electric traps, Winters Howl, great team map
Ascension- 4 90 second death machine's+ high round awesome
MoTD- another great map for solo runs with plenty to keep you busy
Origins- Would rate but xbox is being fixed and probably won't return before I download Origins on Ps3