Mine is Misty. Not only did she add some humor and flavor to the show, her obvious feelings for Ash added a further layer of dynamic; her pulling Brock's ear was a funny recurring gag that lost its touch when she left. Really, after Misty left, the anime fell to pieces because there wasn't the same character dynamic. Misty added so much that May and Max didn't; though May had good development as far as the anime goes, she didn't bring the same compatibility to the table.
I also am fond of Rocket-dan at some points; they have story to them, but once Hoenn hit they threw that all out and instead focused on redundancy in every freakin' episode.
Original series for life.