Of course Saki-chan is the best of the personalities. She seems to have been given the most character development, and she's just funny. Not annoying like Chiyuri. I know a lot of people liked Nanako the best, but Saki is the most divergent from Kozue's personality.

In the manga however, Chiyuri has a really endearing part with Shiratori, which does redeem her character a little bit, but Saki-chan will always be the best.