Oh gosh, haha. The 5th Gen Gym Leaders were all so memorable in general! But yes, from the bunch of them, I'll indeed list my twp favourites!
Like Skyla here. Skyla was really pretty, and you know, I did really like her gym! But yes, sadly, she wasn't much of a challenge if you had an electric pokemon, or heck, maybe she wasn't even a challenge for you in general. Either or, maybe they should make her have some other flying types that would cover her weaknesses, like Gliscor, or something. Either or, liked her very much!
And of course, Elesa! Just like everyone else, haha. I think I preferred her look from Pokemon BW than B2W2, but yeah, she's pretty, and she's actually pretty tough as a gym leader if you don't know how to handle her Emolgas, and that Zebstrika, aah. Like, she was pretty much the only electric type gym that was a bit of a challenge for me, but even then, it took a really good ground pokemon with rock moves to persevere! But that's the thing - you'd need preperation to face her properly, haha.