Erika gets my vote. ^^ She's also my favourite Gym Leader in General. Grass pokemon could be quite great from Gen 1, with their mixture of power-type moves like Sleep Powder, and they weren't too, too bad offensively. Erika back then liked to use Pedal Dance with her Gloom, which would of done great deal of damage, if I hadn't had Charmeleon and Ivysaur of course, ohoho.
Misty is a second, with a cute Mermaid-esque theme. Starmie was quite tough without the proper training, so it was good that the game kept you on your toes and helped you to remember to take advantage of type advantages.