Eh, guys, since when do any of the Elite Four members count as Gym Leaders?
Anyways, from the first region, my favourite Gym Leader would most definitely be Giovanni. There was always something so dark about that character in the game that made him seem like a villian more than a Gym Leader, which, when you look at the fact that he was the person in control of Team Rocket, I suppose he was. The second generation, my favourite Gym Leader would have to be Whitney because Jeremy brainwashed me of the fact that she uses Normal-Type Pokémon and she has pink hair. In the third generation, Flannery steals the crown, unfortunately for the rest. For one, she is one of the best designed characters to ever hit the world of Pokémon, and for a second point, she uses Fire-Type Pokémon. What isn't there to like? My favourite Elite Four member/Champion would have to be Lance. Pink hair plus dragons? Yes, most definitely the coolest character to ever hit the games/anime.
