German Shepherds and Bullmastiffs are both large, muscular breeds of dogs that were originally bred for different purposes. German Shepherds were originally bred as herding dogs, while Bullmastiffs were bred as guard dogs. German Shepherds are typically more agile and quicker on their feet than Bullmastiffs, while Bullmastiffs are typically stronger and have more powerful jaws. In terms of temperament, German Shepherds are typically more outgoing and sociable than Bullmastiffs, while Bullmastiffs are typically more reserved and protective.
With all this in mind, it is possible that a German Shepherd might have an advantage in a fight due to their agility and quickness, but a Bullmastiff might have an advantage due to their strength and powerful jaws. However, it is important to remember that dog fights can be unpredictable and dangerous and should not be encouraged or promoted.