Even though I love Water Pokemon, I don't count Misty since she's partly a main character, and I definintly don't include Wallace (shudder)
My favorite is Sabrina. I soooo want a Psychic Water Pokemon! (Go Team Aqua!)
My reasons for really loving Sabrina is that she's so...evil...I don't really understand how she became a Gym Leader since her wickedness shouldn't be allowed. But oh well, she (unlike almost all of the Johto and Hoenn Leaders) has a story, a great and cool story.
Did you know that almost all of the Gym Leaders of Kanto have a real story instead of "I'm here, battle me"?
Misty: Two sisters who think she's a runt, loves water pokemon and ran away to become a great water pokemon trainer.
Brock: Dozens of siblings(mormon parents most likely) and a father who ran away. A lot of emotion there, no?
Lt. Surge: (Well, more of a story in the game, so Ill use it) Great Army Lutenit, and won all his battles with his supreme electric pokemon. He has a Raichu, but he evolved it from a Pikachu too early for it to learn any good moves.
Erika: Instead of being just a Gym Leader, she's also the owner of a fragrence shop. When she was younger, a bunch of Grimer tried to attack her, but a Gloom came to her rescue.
Sabrina: As a young girl, Sabrina had psychic powers. As she grew, instead of wanting to make friends, all she wanted was to increase her powers, and when she thought of herself better than her parents, she casted them away and split into two: One who honed her powers and cared nothing else, and another, a child; who's only wish is to make friends and be happy, until Ash came and saved her.
Koga: Umm...I'm not sure about this guy.
Blaine: Used to be a hippie and watched as his beloved island became a place for tourists and nothing more. He's the owner of a hotel and trains inside a giant volcano.
Giovanni: What can I say about him? Ring Leader of the Evil Team Rocket, tried to capture the rare Mewtwo twice, and is the source of all poor Jenny's problems.
^^ Whoa, did not expect to write all that when I clicked the reply button! Just a little insight on a few Gym Leaders incase anyone is interested.