I don't know about "worst." The worst individual dog I've ever seen, the only dog I've EVER been afraid of, was an Akita. No problems with the dog, got out of the owner's backyard, sidewalk and front of house were separated by hedges, skulked along the hedges and jumped on the back of a child coming home from school. Dog attacked the child's backpack and then the child, ripping the child's scalp from front to back. I have never seen such injuries. Bystanders tried to get the dog off the child and one very heroic man got a fire extinguisher from his truck, sprayed the dog, the dog ran back to the house. I went to Animal Control (part of my job) to see the dog, and it was literally out of control, jumping and snapping and lunging at the bars of the kennel.
Needless to say, the dog was euthanized. That's the very first dog I've ever been afraid of.
Animal Control held the dog for 10 days and during that time never cleaned the kennel, pushed food and water in using a long stick. Vicious, vicious dog with no prior history. Necropsy revealed nothing.