Pretty much any large dog of sound temperament can serve as some form of protection, but the breed you choose matters enormously when real personal protection is the goal.
Pretty much any dog of sound temperament can be used to some extent as a protection dog, or at least as an alert dog. The smaller breeds (up to 50 lbs) would obviously not be "man stoppers." Any larger-sized dog could be a protection dog, but most dogs must go through vigorous training in order to understand when they are needed and how to be of help.
As an example, police K9 dogs with the highest performance records in trials are sometimes purchased by a police force only to find themselves chasing a suspect in a dark alley and becoming confused because they want to bite, but don't see the familiar sleeve or suit equipment. These cases are rare, but they are evidence of how important it is to select the right training source for your protection dog.
The dog has to have natural protective instincts, be pack-oriented, and/or territorial. Even a Bloodhound could do it in a rare case. Generally speaking, some breeds make better guard dogs protecting their territory, while others make better personal protection dogs protecting their owners and families directly.
Two of the universal protection dog breeds are the German Shepherd and the Belgian Malinois. At present, these are the two breeds that stand out from the rest. It is not an accident that around the world, 99% of police and military dogs are German Shepherds or Malinois.
Doberman Pinschers, Rottweilers, Giant Schnauzers, and Bouviers, among others, could do just as fine a job, but the temperament of these breeds has been compromised in recent years by breeding for looks rather than working ability. The prevalence of genetic illnesses such as hip dysplasia has also risen dramatically in these lines.
When considering other breeds for protection work, it is doubly important to do your homework. We do have dogs of other breeds that excel in family protection and have passed the necessary health screening. If we offer a breed other than GSD or Malinois, you can be certain we did our homework before investing the hundreds of hours needed to develop that dog.
German Shepherds, Dobermans, and Giant Schnauzers — all home raised, all professionally trained.