10. Brock
It’s a bit weird to say that Brock is one of the toughest Gym Leaders since he’s the first Gym battle in the very first Pokemon games. But depending on which starter you choose, those Rock/Ground-types can absolutely crush you.
Good luck to all the Charmander owners in the original Red and Blue because that Pokemon is not budging Brock’s Geodude, Onix, and his five Full Heals. You could catch a Caterpie or Nidoran and grind to level up high enough to overcome the challenge, but that would take forever against the low-level Pokemon found just before his Pewter City Gym.
The far easier solution here is to pick Squirtle or Bulbasaur in the original games. To those who stuck it out and persevered, I salute you and your soon-to-be Charizard for your bravery.
