Which Zombie class is not OP?

I can understand how you feel, and I have a list of characters I usually play as to avoid any guilt:
Soldier:
Pretty much all varients should work. None really stick out as OP.
Engineer:
All except AC Perry and maybe electrician.
Scientist (oh boy):
Not Marine Biologist, Chemist, and Zoologist. A fun varient that isn't Op and takes skill is the base Scientist. Many overlook him but he is pretty good alternative.
Allstar:
I guess Tennis Star, but most Allstar varients are pretty tame.
Deadbeard:
Strong guy overall, but Cannon is weakest. I would definately say don't use Captain Sharkbite.
Imp:
Base Imp and Shrimp Imp are fine.
Super Brainz:
FOR THE LOVE OF ALL THINGS SACRED NO ELECTRO BRAINZ. Other varients are strong due to abilities, so most people feel like Super Brainz is OP.

These are just my recommendations, and remember a lot of players feel the zombies are Op because they do not know how to counter some classes.