As far as a specific dragon goes, the hands down winner is Tiamat/Takhisis. This is probably due to my great love for the Dragonlance books & modules as well as the Dungeons and Dragons cartoon that I watched as a kid. And yes I know she’s a deity and not strictly a dragon, but I don’t care.
As far as dragon types/colors, in general I’m a fan of the more exotic dragons, like the planar dragons. They’re an interesting group with a lot of unique and fun breath weapons to mess with a party of adventurers. As far as the “classic” types, I tended to like the brass and bronze dragons. Small, swift, independent and always speak their minds.