Most difficult Gym Leader battles

The girl with Dragon type pokemon.

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.

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.