The Platinum Dragon has both the longest breeding time of 49 hours, and the highest selling price of $6,000,000. The Platinum Dragon also gives you a whopping 700,000 XP upon successful breeding at level 17.
To breed a Platinum Dragon, you need to breed dragons with the Water and Metal elements. Metal + Water, Water + Copper, or Metal + Seaweed are good examples.
This is a rare dragon with a lower chance of breeding, so keep in mind that it may take many tries to attain. If you get a 49 hour breeding time then you’ve got it.