In my opinion there isn't a most vicious dog, but from what background it comes from or from how it is brought up, or being treated badly. I have seen myself some vicious dogs and they come from bad background.
The most dangerous dogs are those that are not properly cared for, trained, socialized, housed, and fed. Breed makes little difference. Another answer The most dangerous dog to other people that are...
This might sound funny but little dogs such as a Terrier will act more dominent then a big dog such as a Doberman, I for one have owned both at the same time and my Doberman let my Terrier push him...