In my opinion the meanest of the dogs in the world are the Pit Bull dogs.
However some Pit Bulls can be loving dogs but most times the Pit Bulls will snap and when they do they become really mean.
The Pit Bull is one dog breed that should not be around children even though I've seen some Pit Bulls do good around children you just never know when that dog will become mean and unfriendly and attack the child.
The second meanest dog is the Rottweiler which has a lot of attacks on humans as well.