I don't even know why people liked Namine, to be honest. She was annoying and it pissed me off in the game that Sora felt so compelled to thank her, even though she was the one who effed up all of his memories in the first place. UGH, she just makes me angry.
And Xion... ugh. I liked her but her English VA made me rage too much.
Namine definitely wins my vote, i mean come on she is light but darkness at the same time she's insanely mysterious and just has so many things in the story that if they werent there kh2 and beyond would just suck.
Kairi's fine, but she has done nothing yet to make me feel like she's my favorite female character, or one of them, for that matter. She feels more background sometimes, and we need to get inside her head for once and what she feels with certain actions.
I say Xion... I say this because she's Everything I hoped Kairi would become minus the change into Sora near her death... Although she's just a replica, Xion has so far the most Character Development... and so that's why I like Xion