Gym Leaders can either be easy, or they can be extremely difficult to beat. It all depends on the types. Case in point, Whitney is hard to beat without a well trained Heracross. Miltank with Rollout is way too OP! Continuing on with the Johto Gym Leaders, my favorite one is another one of those hard ones. I mean, she is a Dragon type user and the cousin of a Champion, for Arceus's sake! Yep, my favorite Gym Leader is...
Clair!
Now remember, Fairy types were not in Gen 2 or Gen 4. So unless you have a Kingdra, hers is difficult to beat. But hey, I love a good challenge!