This is pretty handy. But you can just google search koi varieties and many sites and suggested books will pop up.
However, whether you think so or not, at the end of the day, it will be about what catches your eye. Koi are like snowflakes--no two are alike. "Good fish" has nothing to do with a specific type. A good fish is in the eye of the beholder at the end of the day. And I've seen plenty of good looking pond "mutts" too.
Of course there are certain standards that experts may like to see in a specific type of fish, and that may or may not appeal to you.
Regardless of type, the first and foremost thing is to buy a HEALTHY fish from a respectable breeder or shop who is passionate about quality care. I have a particular local shop that I buy from because they consistently have good fish at reasonable prices that appeal to me. They also purchase from a KHV (koi herpes virus) tested breeder, so I know that I'm not going to be introducing a problem into my pond. In the early fall I buy from a local koi breeder when he opens his mud ponds to the public for his Fall Harvest.
And, to be frank, your wallet will also dictate things, as koi can be very expensive. There are some who love a 5 inch Petsmart koi for $15.99 and that's what they can afford and they are happy with that. And there are those that want the luxury fish and will pay whatever $ that demands.
Where do you fall in the spectrum? And what are you willing to pay per fish?