I'm not
that into the Kalos Gym leaders, but I guess I liked Valerie and Korrina the most. Kinda liked Valerie's design, and that Korrina does a little bit in the storyline. And I like her version of the gym leader battle music:
http://www.youtube.com/watch?v=CzzS2S1xbuQ
And if I could change one of their types, I'd change one to Dark type. Seriously Game Freak. You should've put a Dark type gym in X/Y. Why is there still no Dark type gym?
As for the Gym Leader that gave me the hardest time....not gonna lie, it was Viola. I almost lost to her (Pansear, why must you suck and not learn a Fire attack until after I beat the Gym?). Everyone else was fairly easy. Especially Ramos. Poor Ramos. My Charizard wrecked him so bad. XD