If you are looking for a pet, get whatever makes you happy. If you want to get into breeding and showing, that's a whole 'nother story!
Make sure they have all their toes, and the correct number of nails on each foot.
Look for dark eyes. Make sure the color is correct, and if they are young, realize that color will change.
A short back, lots of pouff...
Read the standard. Got to the ASBC website. Go to other breeders websites, and look at the pictures of their birds.
Check here on BYC, there is a thread
https://www.backyardchickens.com/t/690632/silkie-breeding-genetics-showing
Talk to people at the show, don't be afraid to ask questions.
Look at the other birds, and try to spot the differences between the winners and the ones that didn't get a ribbon.
It's hard!
And don't get just one....depending on your flock, you will need at least 2 or 3 
Silkies are the mellow ones that always get picked on. They need backup buddies.
But once again, it will be whatever makes you happy.
I have birds with that one crooked toe, or one yellow eye....that are the sweetest.
Have fun at the show!