Most difficult Gym Leader battles

[Alex Boss]

The girl with Dragon type pokemon.

 

[withheldforprivacy]

At Gen1, all Gym Leaders are pieces of cake, no matter what the starter pokemon is:
Brock: Catch a mankey/butterfree and he's toast.
Misty: If you haven't caught a Pikachu, go to Bill's, catch an Oddish/Bellsprout in the way, and she's toast.
Surge: Catch a Diglett at the cave and he's toast.
After that, you'll have plenty of types, so the rest of the leaders are toast too.

 

[Satsuki]

Whitney can fuck off with that Miltank. That thing is hard as fuck to beat. No one else off the top of my head was too hard.