Well in Kino Der Toten I prefer the m16 because of it's location and the route that I run. But if I had a weapon on the wall I would always grab and upgrade it would probably be the mp40. Still one of the strongest SMG's in Nazi Zombies and has a extremely
bigger clip than the rest of the SMG's.
The M16 is more than convenient in Kino Der Toten because of its location on the stage. On the other hand, I prefer the AK74u in Call of the Dead for its location at the bottom of the lighthouse. It really depends on what area you're running. If you prefer
to run laps around the map, you could always conserve your ammo and hit the mystery box occasionally.
I would also have to agree with the previous posts about the M16. If you want to really rack up the points go for that gun. I've used that gun on Kino der toten, Ascension and Call of the dead as my wall gun. In the later rounds you obviously need to pack
a punch it and buying ammo off the wall is a plus. AK74U, MP40 and MP5K are also great guns to tryout as well, not to mention great mobility with the submachine guns!