The gym leader that stood out to me was Dryden's gym at Opelucid city. Just the way to get through the gym and the dragons mad the gym really cool, and after you defeated him he helped you to defeat Team Plasma. He was also a powerful leader, I had a little problem defeating him. His Pokemon (druddigon, flygon and haxorus) were cool and somewhat hard for me to defeat as well. Overall, I thought his gym was fun and was my second favorite gym in the game, but it stood out to me the most. My favorite gym was Elesa's gym in Nimbasa city. As gay as it sounds, hers was my favorite because of all of the fans and the atmosphere just seemed fun! Her Pokemon were easy for my pignite to take out, and you get a standing ovation after you defeat her so that is always great too! Also, the costumes were stylish, and were all unique (even the trainers you have to beat before you get to her).
My least favorite gym would have to be Driftveil's gym. Clay was the leader and his Pokemon (especially excadrill) took me forever to defeat! Also, I had to keep leaving his gym when versing the trainers to revive my Pokemon at the Pokemon Center which took time. I had to do that like three different times. Also, I kept getting lost in the maze, but finally got to Clay and defeated him after a long and arduous battle. Gosh, that gym was hard and annoying but the badge was so worth it because I earned bulldoze which helped my pignite out in many battles to come.
My least favorite gym was probably Marlon's gym. His gym was pretty lame for being the last one. I defeated all of his Pokemon with ease, and I had just wish that getting to him would have been more formidable and that his Pokemon would have been harder to defeat. It literally only took me about five minutes to defeat all of the trainers in Hamilau's gym and then another two minutes to sweep Marlon's team with my ampharos. My second favorite gym leader would have to be Roxie! I liked her attitude, and the rock and roll atmosphere of her gym was convivial and marvelous. It was a very upbeat, but easy battle (though she was only the second gm so I give her an excuse).