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) -