I've played this twice, and the answers for each playthrough are completely different tbh.
The first time I played, the hardest was Charlotte. I'm pretty sure I had to battle her like 20 times before I finally took her down, my team did NOT have anything to take her down and I was debating just quitting the game for good whoops. Easiest was surprisingly Noel, I know everyone finds him insanely difficult, but i swept the team with a Magcargo (got a couple shell smashes off immediately and swept). This first playthrough the later gyms were all impossible because I refused to switch any of my team out so I was in the 70s with a team of six i got before the second gym including a Magcargo and a Pachirisu, the second I finally beat Ciel and got told I got to the end I was so relieved, the torture was finally over...
The second time I played (my current save), Shelly ended up being the hardest, which I did not expect at all! I remembered her being easy the first time, but the second time playing I just had nothing to use against her. Ciel was a close second, Mega Altaria is still a nightmare. Easiest was probably Luna?