I gotta give some love to the Dragons of Lunar. White Dragon Quark literally shat diamonds, and the baby dragon characters of the first two games (Nall and Ruby) are just great fun to be around. NPC: "Ghaleon has become an omnipotent man!" Nall: "Eww! Why would you tell us that? We don't care if Ghaleon shoots blanks!"
But you might argue that those dragons are too soft, too minor, and that's fair. May I introduce you, then, to Nicol Bolas, of the Magic: the Gathering lore.
He is one of the oldest (possibly THE oldest) and most powerful beings in the Magic multiverse, though his power has been drastically reduced lately due to a shift in the nature of the planes. Finding himself mortal for the first time in millennia, Bolas has begun a scheme across several universes to reclaim his lost power. Despite his enormous power, he prefers to manipulate others to enacting his will. To him, there is no difference between victim and servant, and while his enormous pride might be his eventual downfall, it will take multiple planeswalkers working together to overcome him.
Plus, in his original art on his original card, he was sitting in a skull-adorned throne reading a book, with a stack of books beside. How can you not love a well-read dragon?