This answer can greatly vary. Any breed of dog can be trained to be kind, loving, and sweet. However from previous life experiences I believe the meanest dog is a pitbull. My next door neighbor has...
There is no naturally mean dog, it all depends on how the owner trains them. Some dogs may need more work but breeds such as Pitt bulls , German Shepherds, Chowdren/Chow Chow, Rottweilers and others...
If you want to speak technically, there are no "mean" dogs. There are only dogs that have a somewhat violent or aggressive nature. These dogs would include, on a dog-to-dog basis: Pit bulls German...