There is no one breed that is better to show than others. Some breeds tend to have a better temperament than others. Just look for a good heifer, that suits your capabilities, and don't worry about winning. It's all an experience and you should look at it as a fun exercise, not a competitive one.
soundness: back feet should track exactly where the front ones were.
capacity/volume: good spring of rib
width:from the back AND front
length: long level topline
femininity: girlish looking
hair: lots of it
I know I left some out. They're in no particular order
What type of show are you going to take her to if you take her at all. some don't need tons of hair if it's not a verry competitive show. but if it is a good heifer and she is the best she can be for HER breed she won't always need a lot of hair. JMO hair just helps. Got with what Cowgirl said. You also want to look at thier feet make sure that they don't toe in or out. Make sure that thier legs have Flex but aren't cow hocked or too straight.