Xion for me. Though Kairi is getting better. Aqua would be second...in fact I'll just make a list for the current cast.
1: Xion
2: Aqua
3: Kairi
4: Ava/Namine
5: Olette
6: Skuld
7: Invi
8: Streliztia...or however you spell it.
I don't dislike any of them, but some just feel kinda there for me at the current time.