Extremely large dogs are prone to bone defects, a short life span and heat intolerance. Extremely small dogs have mouth problems and delicate bones and joints. On a more practical level, toy dogs are usually not good with children who they can find threatening, and anything over about 25 Kg eats a lot and is too big to fit in the car comfortably. By the same token a highly intelligent and extremely active working dog can be totally unsuitable as a pet unless you live on a farm. My ideal dog would be trainable without having a strong working drive or being overactive.
Appearance is always in the eye of the beholder.
Some people want an aristocratic fashion accessory, some a knockabout backyard dog. But most people want a dog that appeals to their eye and they want to know what the puppy they have acquired will look like as an adult. My ideal dog would have to be predictable and appealing in appearance and an attractive colour. An afghan cantering on the beach with its hair flowing in the wind makes a wonderful sight but, as many people found when this breed had a burst of popularity, the long coat matts easily and requires constant grooming. Labradors and German Shepherds have a heavy undercoat which sheds profusely in spring and needs constant grooming at this time.