May over Misty any day...why?
In the beginning May caught my attention...her self-introduction was very unique and probably started the character development I love.
Her quote from the first episode she was in "The truth is...I don't like Pokémon..."
At first, she had no idea what she wanted to do in life, and she didn't really have a thing for Pokémon at all.
I like how she explained that she was uncomfortable being in the Hoenn region and that she just wanted to travel to see the world.
She didn't know how to command Pokémon attacks or types or anything at all...but once she saw a Contest, she immediately discovered her passion for life, and by the end of her journey with Ash, she was able to look back and say "look how much I've grown!"
Misty really didn't have too much development other than feelings for Ash and frankly, she was around too long, so I prefer her cameos to be just about as frequent as Gary's.
Both were very good supporting characters, but May stands out as the one who "found herself" to me.