If we're going by the games, it'd be Norman because of his relationship with the player and how he gives the impression of both a strong trainer & a wise mentor.
If we're going by the anime, it'd be Tate & Liza because of their sibling dynamic and their match with Ash was pretty good (whether or not you take Thunder Armor into consideration)
If we're going by the Adventures manga, it'd be Roxanne because I love how it portrays her as a stuck up honor student who only accepts challengers that pass her written test. It fits perfectly with her gimmick & puts a unique spin on a classic anime trope.
Norman for sure. I mean he's our dad in the games and who doesn't love their dads.
Jokes aside, I think he was a pretty Solid gym leader, in fact I really had a tough time beating his second Slaking lol, man it was just hella strong lol.
Being my most appreciated character in Pokémon, Flannery is certainly the gym leader I'm going for here. I find her hairstyle to be freaking amazing first of all, as it makes me think about a red painted star; her gym, despite its "brain teaser" which I guess for most people can be a bit annoying, is a fantastic place since it's located on the site of a thermal spring; and at last, I like this character because of her all around messed up personality.
On a side note - her signature Pokémon(s) are Camerupt and Torkoal, both of which I happen to make use quite a lot... and it's been since Ruby, my first ever Pokémon game, that I've always dreamed about one thing: being able to walk through the various Pokémon regions with her sprite (let's say just up to Gen. V at least, when the main character couldn't be customized) -
Norman for me as well. Your battle with him was built up from the start and he’s the main obstacle till that point. He’s also literally the only dad we’ve seen from a player character. He’s reasonably challenging too, especially back then when I didn’t know how truant worked so couldn’t exploit it.
It’s a toss up between Liza and Tate and Norman. All of them are interesting and I like the backstory behind them. The twins are interesting because of the urban myth that twins can read each other’s mind and in Emerald, they pretty much cover their Aces’ weaknesses pretty well and utilize the Double Battle mechanic very well while not being a hinderance to each other at the same time. It’s pretty surprising that a Double Gym Battle would not happen again until Raihan in SwSh. Norman is very intriguing as he is currently the only player character father to ever be seen in the games and you can actually battle him. I like how his Gym really doesn’t have a traditional Gym puzzle and the real test is to see how well you can adapt and overcome any single stat being altered by an item. You actually need to think outside of the box when facing the Gym trainers. And then Norman’s Slaking will definitely throw you off course by having Facade on both of them in RS and Counter in Emerald which both can hit hard under the right circumstances. And in ORAS it’s even worse since Slaking has Retaliate on all of his Pokémon which will severely hurt you as Vigoroth and the other Slaking will immediately use it as soon as they are sent out.