Unova has some of the most awesome Gym Leaders in Pokemon ever, but here's the ones I loved out of the crowd:
I'm one who usually doesn't find Poison-types appealing, but Roxie somehow makes the type look awesome. The fact she's a rockstar and a Gym Leader in one definitely makes her one of my faves, and I think I started liking Poison-types a bit more thanks to her.
Elesa is literally my fave of all the Electric Gym Leaders, along with being the only girl of the five (with all those males you'd think this was a type that appeals more to guys than girls). She also uses Emolga, which is my fave of the Pikaclones and in B/W at least, she does prove to be a decent challenge. Her redesign in B2/W2 made her even more flashy but damn she's beautiful.
Marlon. Oh god, Marlon. This guy is literally my fave of all the Gym Leaders, because not only does he have some pretty sick hair but he also uses one of my fave (non-Oshawott line) Unova Water-types, Jellicent. He's like put a total cool image on the already-cool Water-type.
Most Dragon-type trainers give off a badass appearance, but Iris is the first one to actually have more of a cute appearance while still keeping the badass image of the type. I was also one of the very few people who actually liked her in the anime, and I never get why she was so hated, but I also do like how the design of her hair is heavily based on iris flowers, which explains her name hahahaha.