In my opinion, Gym Leaders in XY were little boring : P I'm not really fond of any of them. They were not that annoying, but nothing too special either. However, I liked how some of them, or at least Korrina, had bigger part in the storyline. And their designs were somewhat unique, which was also very nice to see. But if I had to name a favourite, I'd probably said Ramos. Because who wouldn't love old man with gardening scissors almost his size 8D I also liked the way he spoke to player in the game. And the grass type is my favourite, and the gym was somewhat fun to go through with vines hanging around. The Gym itself was relatively easy one (I started with Fennekin), and I liked the background of the battle with books and stuff.
Among Elite 4, Siebold is my favourite ♥ I really like his design, and there's just something in him I can't resist. Probably it is the artistic side of him? Can't really say. I like the other members as well, and I really like it how you can meet them in the Battle Maison as well. They feel more closer somehow. Wikstrom's armor is amazing, and Malva's background with Team Flare is also interesting.