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This is a very complex question. Traditional breeds for service dogs have been German Shepherds (GSD), Labradors, and Golden Retrievers. But nowadays the use of unusual breeds has exploded. Mastiffs are used for mobility work. Chihuahuas are used for diabetic or seizure alert dogs. If the dog has the temperment, skills, and willingness to work; almost any breed could do certain jobs. A corgi wouldn't work out for pulling a wheelchair but but could work as a hearing dog. Breeds like pugs and bulldogs don't always make the best of service dogs due to the pushed in noses--this leads to difficult breathing while walking and a shorter working life. While toy breeds can do some service dog jobs, they are not often taken seriously by store employees and the public, especially if dressed up like someone's child.
Smaller breeds are being used by more disabled people on a fixed income as they eat less and can live happier in a smaller home. A cocker spaniel can alert to a sound just as well as a labrador.
Bully breeds, dobermans, and rottweilers are used as service dogs. This can caused access problems in areas with breed specific legisislation (BSL) aka breed bans. Some cities require service dogs of a banned breed to be muzzled in public. Or you may not be able to purchase a banned breed if you live within city limits.
Housing may also be an issue with a banned breed, or a breed considered "dangerous." In some cases, a landlord can refuse to permit a dog of a certain breed on the premises. See Can a landlord refuse a service dog based on breed?