Honestly, Bugsy is always one of the Gym Leaders I mention when I talk about my overall favorites, just because he's so smol and cute yet really challenging to beat (at least, if you were cool and picked Chikorita). Falkner gets to share that spot with Bugsy, due to being the first Gym Leader I ever fought, and for having a really good design. Morty, Clair, and even Jasmine are up there, too, for having cool designs and being cool characters all-around.
My least favorite... is probably Chuck? Nah, he threw a rock in the air with his bare hands... Uh... I guess Whitney then. Not because of her Miltank or anything, but because she cries and doesn't give you the badge you earned right away. I mean yeah Clair doesn't give you your badge right away either but that's on purpose; Whitney just straight-up forgets.
I'm fairly indifferent to Pryce. I think his character and design is handled better in the remake.