Cocker Spaniel
Most experts agree that English Cocker Spaniels are not suited for guard dog duties, as they are typically not aggressive enough. This is largely due to their loyal and friendly nature towards humans, which makes them better suited for companionship and other non-threatening roles.
Boxer
As the AKC Boxer Breed Standard from 1938 summarized perfectly: “His intelligence and willing tractability, his modesty and cleanliness make him a highly desirable family dog and cheerful companion. He is the soul of honesty and loyalty, and is never false or treacherous even in his old age.” Anyone with a boxer will tell you how true this is!