Some people say the best breed of dog isn't a breed at all. The American Temperament Testing Society's evaluation measures stability, shyness, aggressiveness, friendliness, self-preservation, and protectiveness toward the dog's handler. Of all mixed breeds tested, 85.4% passed. That's well above the percentage of such popular purebreds as the Poodle, Sheltie, Schnauzer, Beagle, and Cocker Spaniel.