Good Koi are judged based on body shape, pattern, colour and elegance, the body shape being the most important.
They should be symmetric in shape and clear of any deformities.
Their pattern should have no blemishes and the edges must be crisp and sharp. If they have stains on their face, head or fins, they are less valuable. The colours must be deep and pure with sharp kiwa(edges of the patterns. If the Koi swims graceful and with elegance, it is more valuable.
On the other hand, most of us are not interested in show quality Koi. Beauty is in the eye of the beholder. Are you buying Koi for the Judges, or for yourself to admire. Why should I pay much more for a Kohaku with no red pattern extending over the eyes, if the one with a pattern over the eyes is more beautiful to me.
Part of the whole experience is to be able to choose a young Koi, and to see how it develops into a beautiful creature. Remember, the way you treat the Koi, will have a great effect on how it will turn out. It is an art to transform a Koi into a graceful living jewel, and it is not only depended on what you bought. Bloodline plays a major role, but nutrition and water quality have a great influence on the development of the Koi. Even luck plays a major role, as one can not always predict how a Koi is going to develop. The older the fish, the less it is going to change. Therefore, there is a huge difference in price between a six month old Koi as apposed to a twenty four month old Koi.