I do think that dogs are man's best friend in many ways, one of which being the defence of their owners. Man has had domesticated dogs for about 100,000 years, so we've had a long time to adapt to one another. We do live with our dogs in a pack formation and, as dogs still retain that instinct from their wolf ancestors, this is why they tend to protect and defend their human pack members. There isn't another domesticated animal that even comes close.
While one could argue that we simply assume dogs to be the best candidates for protection because we are not currently using many other animals this way, my feeling is still that we are correct in this assumption. I feel the bond formed between man and dog is remarkably strong which assures the dog will do anything possible to protect the man.
Yes, dogs are the most successful domesticated animals for protection. There are no other animals that it is safe to keep for protection. Other animals are not domesticated enough to have for protection. Animals such as wild cats may pose a danger to the owner and others. Dogs are the only domesticated animals for protection that are not too dangerous to have.
Domestic dogs are descended from wolves so recently that they remain wolves in all biological essentials, including their social behavior. They are territorial.
They hunt cooperatively. Pack members are emotionally bonded and greet each other enthusiastically after they have been separated. Dogs acted as human's alarm systems, trackers, and hunting aides, garbage disposal facilities, hot water bottles, and children's guardians and playmates.