Really to me, having grown up as a 4-H kid that always showed poultry, I think it is a mistake to purchase based on what you think may win. IMHO, *always* buy what you are going to most enjoy. Work with the birds that you have the most genuine interest in, not what you think has the highest likelihood of winning.
The fact of the matter is- it is impossible to know what is going to do well on any given day. Any breed has the potential to win. That said, in general, the older more established breeds/varieties generally are easier to find show quality stock in. It is really a crap shoot though. There are so many potential variables.