Kanto: Blue, he's the only gym leader to not stick to one type, and seeing his development brings a nice close to his character arc.
Johto: Jasmine, her design is my favorite (in layman's terms: I find her the hottest) and find her Steel-type specialty interesting for some reason. I also like Whitney a lot, probably because I didn't have a hard time the one time I fought her (haha go Slowpoke!
)
Hoenn: Flannery, specialising in fire-types results in an immediate bias. Her personality and design are also a plus.
Sinnoh: Candice, her personality and design are just so contradictory to her specialty (how the hell does she survive with that skirt!?)
Unova: Elesa and Skyla, mostly because of their designs
. That and Skyla's complete disregard for safety precautions.
Kalos: Korrina, she's the most story important of the Kalos leaders and she's got a unique design amongst Fighting specialists (skater instead of karate specialist).