His Excadrill is the Excadrill that shall pierce the heavens! can and will destroy your team if you're not careful. There's also the fact that he's the "Underground Boss", which is an awesome title.
Elesa gets points due to the fact that I had to train up a Sandile just for her, and even then, I still struggled against those damn Emolga.
Elesa for having arguably the most devious strategy out of all the Gym Leaders in the Unova region, as well as having the most amusing Gym design (Skyla's cannons are a close second). And for being completely awesome.