I haven't hated many of the Gym Leaders. I think Lt. Surge and Lenora are both rather stereotypical and offensive, but if I had to pick one to actually remove from existence, it would have to be Roxie or Marlon. Marlon was pathetically easy and just an annoying experience, especially as an 8th Gym Leader.
"I'd tell you to buy a clue but you don't have enough cents."
Clay. Surprised no one else has said it yet, he's pretty much a jerk every time you see him. Blames you for his losing Team Plasma, doesn't give you your TM until later, bosses around everyone (including you after you beat him), claims ownership of vast swaths of land which don't even seem connected to him, names a tunnel after himself. Plus, if you don't have the right team, his Excadrill can rip you apart. (Never actually been a problem for me personally (I always pack a water type), but still a reason to dislike him)
Seriously, the only Gym Leader I've ever hated, and Morty owned me like 40 times as a kid.
I believe that everyone should be nice to each other on these boards, or at least be respectful or only jokingly mean. Number of people who agree: 45