I have a few favourites, depending on what the setting requires.
Black dragons are the most dangerous to fight, because the PCs must confront them in an alien environment that the dragon can make even more dangerous by digging holes and spreading acid on the water surface. They are also the species that lairs closest to human lands. I'm not a fan of the sheep's horns look though.
Blue dragons seem the most rational and calculating, and work well as cruel masters of desolate and faraway places that the PCs must journey to reach.
White dragons are moderate in strength and more animalistic, making them useful as parts of strange ecologies and interesting social orders, such as tourist hunting lodges.
Reds seem to be the most devoted to evil cults and demonic or devilish masters, and the most likely to be fiendish.
edit: I like the idea of making dragon scales difficult to recognise by colour, using unexpected races in different terrain, and mixing up their breath weapons (or giving all races burning, fuming venom as a breath weapon). A dragon should never be merely one of a breed.