This is an easy tie. I can’t choose one over the other.
Brainstealer dragon. Take a dragon, make them also a mind flayer. Epic, massive, brilliant, and a devastating NPC or villain.
Draco Lich. Take a dragon, make them also a fully decked out lich. How is that not the most intimidating dragon in the multiverse?
The power and roleplay possibilities with those two dragons are seemingly limitless. Also, they look badass.