A huge and difficult question to answer! Choose something that is easy to breed. Some breeds require double mating, i.e. seperate pens of birds to breed males and females, there are others where the colours are so complicated that it takes years of careful breeding to get them to perfection. Breeds with feathered legs or crests for example, are very high maintainence as for show these have to be kept indoors on deep shavings with special drinkers to avoid getting their feathers wet. It must be a breed that you find attractive and that is within your capabilities to manage. I think if i was starting showing now from scratch, I wouldn't choose the breeds I now keep - which have just 'happened'! It is impossible to give a straight answer of a breed to a question like this without knowing your situation, however examples of good show birds for beginners would be Sussex, Dutch Bantams, or Orpingtons. These are hardy, easy to manage breeds (from a practical point of view), which will be readily available from numerous sources (as they are reasonably popular breeds) and are beautiful birds which are a great introduction to the joys of poultry showing! I hope this is of some help to you!