The gym leader I had found to be easiest when it came to defeating would be Erika.
By the time we do get up to her, a lot of our Pokémon manage to reach a decent portion of level, even if we don't put much effort under training, which would be one reason behind this. On top of that, her team doesn't seem to know much hard-hitting moves, either; had they been different, I'm positive her difficulty would've been slightly more high.
Aside from her, I've also found Gardenia to be easy due to similar reasons; unfortunately, not many of her Pokémon had actually known much damaging attacks. As for most difficult, there are a few, with one being Whitney. Her Milktank was always a big challenge to take down.