I\'ve done some testing with the different classes and found that Necrotech lab assistants have it easy. 4exp just for using the DNA extractor on a zombie!
Oh, and zombies have one of the hardest times leveling up, considering that the fleshbags only have to close the door to keep us out, and attacking one of your own (zombie vs. zombie, and living vs. living) only givs you half as much exp as attacking someone of the opposite side of the food chain.