of the games I've played, I'd have to say: Cynthia's the hottest Champion, Karen for Elite Four (excluding the champions), but for gym leaders, Erika, Sabrina, Jasmine, Clair, Roxanne, Flannery, Winona, or Candice. If I had to pick, maybe Flannery.
When I was a kid, I had a massive crush on Erika. She's still one of the hottest female Gym Leaders to me, followed by Whitney and Jasmine. Kamitsure is growing on me too. So is Fuuro, but she's a little too blatantly sexified for my tastes.
As for the males, Morty. And Lance. And definitely Volkner. The Sanyo brothers too, especially Pod.