There's many gym leaders, but here are my favorite:
1. Johto League: Falkner (flying) - because of an epic gym battle, Charizard vs Pidgeot
2. Hoenn League: Vinona (flying) - I like her (battle)style
3. Unova League: Drayden (dragon) - he's such a badass, and his look reminds me of Hannibal mask somehow. :p
I didn't think about my top3 less favorite gym leaders list, but all I know is, that on the top of it would be Kalos League gym leader Valerie (fairy). I think she's the most lame gym leader of all times because of her weird battle style, appearance, and also... "fairy".
"Homo sum, humani nil a me alienum puto"