Which gym leader do you dislike the most? Is it because of their design, team, what?
Time has eroded my ill feelings for most the older gym leaders, so I'd probably go with Skyla. I'm still unsure what that thing coming out of her head is and light blue does not look good on her. It also doesn't help that I can't even remember that Swanna is a pokemon.
Clair. The definitive sore loser who makes you jump through added hoops just to get a Badge, which wouldn't be so bad if not for the fact that she WANTS you to fail so that she doesn't have to admit her loss.
Whitney. Not just because of her miltank, because of her over-childish personality and the fact that she CRIES after losing a battle. Suck it up, Whitney!
She's a stereotypical girl in every sense of the term, so of course she's gonna bawl her eyes out when she loses. At least Whitney is a good sport and gives you your Badge once she calms down.