I used to be a vet tech and the breed that scared us more than any other was the American Cocker spaniel. They were biters almost without exception. Scottish terriers and dachshunds were close behind, and most Chihuahuas could not be trusted. Occasionally, we'd get vicious shih-tzu, which wasn't so bad except you can't wrestle with a long-backed dog (you can hurt them) and shih-tzus have no nose on which to put a muzzle.

Shar-peis, chows, and Akitas were probably the most dangerous but at least they were big enough to grab onto without fear of injuring them.

I finally came to the conclusion that the kinds of people who own really nasty pit bulls, Rottweilers, American bulldogs, dogos, etc. are the kinds of people who don't take their dogs to the vet, because we sure didn't see many of them. We had a Rottie patient that we had to sedate just a bit because he would go into "puppy mode"--he was 98 lbs. of solid muscle--and wouldn't stop wiggling and trying to lick everyone.