A difficult question, especially if you're like me and you appreciate all kinds of cats. In which case, even the pure bred cats will leave you chooosing between 40 different breeds of cats. Breed aside, you will want to see how well you get along with the cat, check how adjusted it is to humans.
Actually, when it comes down to your final decision, you will probably care more about how much connection you make with the cat and not so much it's breed. This is an important part of a cat's development which falls partly on the breeder. Indeed, the personality of the cat is cultivated by not only the mother, but also the breeder. Frequent contact with the kitten will help it adjust to being around human beings.
For this reason, it is important to try pick your kitten from the breeder directly, and not from a pet shop. Kittens at a petshop have dubious origins and very little human interaction. This can lead to mal-socialisation.
Health wise, kittens from a petshop cannot be guaranteed to have health problems, particularly genetic disorders. Professional breeders will selectively breed to avoid genetic disorders. It sounds like a good idea to choose cat's with no health problems but if you're looking to pick up a kitten from a shelter, then the moral advantage of that may outweigh it's potential health issues down the line.
Kittens should have no fear of people, and this is the best indicator of a well socialised cat. You should also look out for lethargy and runny eyes and noises, these usually mean the kitten has some sort of sickness. In general, kittens should be very playful and full of energy!
