Looking at the poll made me realize I like all of them really.
But if I did have to pick one though, it would have to be Lenora. I'll admit, it's partially because it's nice to see a Gym Leader and a Pokémon character in general that's married, but she also freakin' owns a museum. Which basically means she's pretty much interested in archaeology, anthropology, paleontology and mythology, all of which I'm interested in so that's pretty much why I like her. And well, as much as I like Dragon-types, I think Normal-types are underappreciated so it's nice to see one. Also, she doesn't mess around in battle. Sure, she's just the second Gym Leader, but don't take her seriously and she will take you down.
The others that stand out to me are for similar reasons. Iris and Drayden for being Dragon-type Gym Leaders, with the former for becoming Champion and the latter for usually being depicted as being able to stand toe-to-toe with Pokémon. Same with Brycen since he's an Ice-type martial artist, with the added caveat of the amusing fact that he's also an action movie star. Clay's also cool for me since, well, he's a business man and he's Gym Leader. Authority Equals Ass Kicking in my opinion. That and he looks like a cowboy.
I like Cheren in that we actually got to see a good bit of his journey to become a Gym Leader because he was our rival in BW. Admittedly, I only really like Cilan because, well, of the anime. Always did like his personality, personally. That and I like Simisage the most of the three monkeys. And lastly, Roxie. I like rock music, I like Poison-types and I love the fact that she's a rock star who uses Poison-types that's a girl. Too bad she's in Gen V otherwise I could've easily imagined Toxicitry would've been her "main".
The rest I like, but not quite as much as the ones mentioned.