When I say worst breed I am not saying that these dogs are evil. Just that they are not my favorite. So to answer your question, my least favorite are dogs that were originally bred for fighting. That's just because if they get into the hands of someone who lacks basic dog training knowledge, or someone with malicious intentions, then there's the biggest potential for real damage. So I am not an advocate for promoting these types of dogs as pets. I think it's fine and good if you adopt one from a shelter, and then have them sterilized, but to breed a batch of pitbull type dogs to sell as pets I find to be unethical, because like I said most people do not know how to handle these dogs properly.
My favorites are companion dogs, just because I think they have the best temperaments for pets. A Yorkshire Terrier with a negligent parent is a far smaller threat than an Akita or American Pitbull Terrier with a negligent parent.