That depends on what you are looking for and what your preferences are, and what you are equipped to keep. Do you have enough room for Standard size fowl, or do you need bantams? Do you like ornamental fowl, or laying/meat type fowl? Do you like something a little more docile, or something with a little more energy? Do you want to show something with a lot of competition like Old English and Moderns, or something that isn't so common?
PouletsDeCajun is right. Also, how much work do you want to put into your birds? Certain show birds require more care to be in tip top show condition than others.
If I were you I would attend a few shows to look at the birds and see what types and colors you like best. Chickens come in a fantastic variety of colors, shapes and sizes.....they really are quite wonderful that way. Some birds are more challenging than others, so ask lots of questions about the birds you find attractive and start to narrow your choices down from there.
What birds you should show isn't something we can tell you really, rather it's something you should discover for yourself.
If you narrow your choices down to a few let us know and we can give you feedback on the temperaments of the birds and any particular tips and tricks are involved in raising and showing those particular birds.
No breed is easier or harder to show than another. Some are easieer or more difficult to keep in condition. Find a bird that you like--if you don't really like a bird compared with a different breed or variety, you likely won't spend the time needed, even if it is less, and you may grow to resent the bird for not being what you want.
Better to enjoy the bird you choose, even if it takes more time.
I have to disagree with "no breed is easier or harder" Some have color patterns that have to meet standard.. some like an all white bird for example you only have to deal with breed type and confirmation while a Golden Lace for example you add the perfect color pattern to the mix. A clean legged bird doesn't have the worry of the feet feathers being worn etc.. So there are easy and hard breeds and color variaties.