Chihuahuas, Pomeranians, Miniature Pinschers are some of the most aggressive ones I can think of. Most vicious dog I ever knew was a Golden Retriever.
Check out Nunu. He was Cesar Millan's most vicious dog so far and he's a Chihuahua.
The dogs that are more costly to insure are considered Black Listed Breeds and all insurance companies except State Farm will ask ridiculous premiums and outrageous rules (the dog needs to be chained within an 8' perimeter fence) if they don't drop you outright from your policy.
These dogs are Rotties, Amstaffs, Staffordshire Bull Terriers, Bull Terriers, American Pit Bull Terriers, Cane Corsos, Presa Canario (sp), Huskies, Malamutes, Chow Chows, German Shepherds, Doberman Pinschers and I think Dalmations are also on the list now. The "Top Ten" (since there used to be only 10) are stereotyped as aggressive dogs because the media spotlight landed on them. It was Dobies in the 80s, Rotties in the 90s and Pits have managed to hang onto the spotlight for the rest of the time. Irresponsible ownership makes it into the news. Good dogs of those breeds don't and neither do aggressive Golden Retrievers that attack people.