Oh sorry, I meant of the Hoenn gyms which was the hardest, sorry that I didn't make that clear lol
You could do it for every region ;).
For me, based on region, it?d be the following:
Kanto: For me, it?s always been Misty if you can?t take down her Starmie fast enough. Not only does she love to turn it into a Physical Tank via Harden and X Defense, her Bubblebeam(or Water Pulse in FRLG) can hurt you quite a bit at that point in the game. An honorable mention is Koga and Giovanni in Yellow version. Double Team is an underrated move and if you allow either Gym Leader to use it(Koga and his Venomoth while Giovanni has his Persian), you?re in for a painful match.
Johto: A lot of people will say it?s Whtney and that Miltank but for me, I?ve never been fond of Gym Leaders with Status moves and Clair is that Gym Leader. Her first two Dragonairs both know Thunder Wave and Paralysis can just ruin your day. Honorable mention is Morty though. Ghost types can really cause problems if you don?t have a Super Effective counter.
Hoenn: Tough one but I?ll go with Wallace. Milotic is a pain for a reason, but honorable mention for Tate and Liza(Double battle with Psychic types).
Sinnoh: Fantina for me. Never been fond of using Normal Types in my team and I struggle to even use Dark Types(which counter Ghost).
Unova: Iris, be it Champion or Gym Leader. Girl just knows how to beat your butt if you don?t bring Dragon, Ice or Steel Types.
Kalos: Valerie... new Fairy Typing is a force to be reckoned with.
Alola: Acerola... even if she?s Elite Four, this Loli just kicks some butt if you let her(would make Agatha proud).