I'd still have to rank Haruka's arc as number 1, with Kaoru's following second. Ai's was good, though there were quite a lot of sloppy scenes, making the pacing pretty bad. Guess it doesn't help that i was comparing it to
Love Goes On, which is superior in every way. It's just that you don't hear Yukana while reading.
Rihoko's should be good going by what i've read about her arc. It'll be my favorite arc for one reason though. :3