Firstly a stallion is an uncastrated male of any breed of horse. Next, Palomino is a color and not a breed, palomino coloring can occur in any breed with the cream gene. Now out of the three actual breeds, Mustang, Shetland, and Appaloosa, the Appaloosa would be the must popular in terms of numbers alone. However many people would favor the Mustang just because it is a symbol of freedom, power and the old west.
personally I like shetlands since mustangs are wild. palominos and stallions are not breeds, a stallion is a male horse over the age of 4 and a palomino is a colour.