Discus are generally very pretty, but they also cost more than most other fish. First time / prospective discus buyers are usually overwhelmed with the many different color / pattern strains and often go on emotional response as to which fish to get. They also usually go to their local fish shop (LFS) to purchase their first discus. Unfortunately, many times the store employees are not experienced in keeping discus and will unintentionally give the wrong information. To make matters worse, the discus at many LFS will not have been kept in optimal conditions either.
Here are some photos to help you choose/ select a good fish - regardless if you choose your fish from the breeder's fish farm/ hatchery or a good LFS. As mentioned before, do your homework first. I've seen several newbies/ beginners decide to jump in and get discus only to find out they have ended up with a lot of problems (sick or dying fish usually).
I've also seen many beginners proudly show their newly acquired fish off on forums, only to be told by more experienced discus owners that the fish isn't the "beauty queen" that they thought it was. Embarassed, the beginner will say, "I'm not looking for a show fish" and sympathetic people will say, "if you're happy with it, that's all that matters"... which may be true, however, if you've spent good money on a fish that is supposed to look pretty, shouldn't you get your money's worth? Also, shouldn't you get a fish with the potential to grow out to is fullest maturity and look nice?
Finally, no matter how pretty the fish is or how much you want it, do NOT buy it if it's sick and don't buy any of the other discus sharing that same tank. Period!