I don't think that there is a best show bird breed,
just whichever you like best is the best. Although I would have to admit some are easier to show than others, for example a crested breed compared to a non-crested breed and a feather legged compared to a clean legged.
Start watching the show results and see what does the most winning in your area.
If you are interested in ducks, Calls and Runners do exceptionally well. For geese, the Giant Dewlap Toulouse shows well (just expect to pay huge money for a really top specimen)