I do agree about the owners, some do it to look hard, some just don't socialise them as pups. The angriest dogs in my experience are chihuahua's, Yorkies, Shi tzu's and Jack Russels all little dogs but snappy as hell. Staffies are actually very friendly and great with kids but if taught to be aggressive they will be.
I've met Rottweilers who had some of the best temperaments I've come across and they were labelled dangerous at one point, seems to go in fads which dogs are to blame.
The only dogs that make me nervous are German Shepherd's they are very unpredictable and i don't trust them at all, especially when used as guard dogs.