I'm going to guess that Discus are your best bet. You can make good money breeding rays, but the cost to feed them and the cost of some huge to keep them in makes them not so great on the wallet. It also depends on your area as to what will sell.
this is a good point for exmaple in my area rays probably wouldnt be to good! i would be able to sell them to some stores but they would have trouble selling them so they wouldnt want too many more! but cichlids are good roun here ecspecially discus!
If you really want to make money in breeding, you can't have just one fish. You also can't limit yourself to selling only to aquabid buyers, or other sites. Discus are ok, but remember that more often than not, you won't sell half of the young. I have also found that discus (especially for breeding) cost more than any other fish I have had to raise.