Mastiff breeds are very territorial and protective. They might not be aggressive in all circumstances but they can be triggered and change demeanor on a dime. All mastiff breeds are derived from the molossus war dog. The lists of most dangerous dogs are heavily populated with mastiffs: Tibetan mastiff, Pakistani mastiff (bully cutta), South African mastiff (boerboel), Caucasian mastiff (ovcharka), and other mastiffs. I'm not sure how you are defining vicious but mastiffs can be aggressive and dangerous, particularly in the hands of inexperience dog handlers.
I define vicious as unprovoked attacks.
Guess I’ve been around too many mastiffs in day. The and the coonhounds.
The only dog I have ever been bitten by was a Mastiff. He wasn’t aggressive, just walked calmly up to me, and I was thinking I had it made, once I passed him and he just walked up to me and bit me hard right on my gluteal.... then turned around and walked calmly back to his house.