Well, I guess I'm not being original here by chosing Sei, but she's definitely my favourite character so I'm following the crowd...
I just love how we got to see right from the start her playful and funny side, because at the beginning we don't know much about the others and they seem distant, and the atmosphere is somewhat austere, so she acted as a great comic relief in the earlier episodes.
Later on, we get to see what happened between her and Shiori... I must confess episode 11 is my favourite, I'm such a sucker for love stories... even more so when they end sadly.
She's as great as ever in ~Haru~. I especially like how her relationship with Shimako is depicted. Of course it's a pity she graduated, but she's not very far away, as is proven when Yumi has tea with her (I kinda like the new haircut BTW). But it's a new season, time to discover new characters, so I guess old ones had to go eventually. Actually I'm kinda glad graduation doesn't take place until episode 6 or so, I thought it would happen earlier.
Oh, and she drives a New Beetle too, my favourite car ! Another good point
(and she's an horrible driver, just like me...)
And, of course, I just LOVE how she makes Sachiko jealous by jumping on Yumi
I also like Yoshino (because of the tomboyish attitude, she's so funny !) and Shimako (the only girl that looks cute, in my opinion... girls drawn in a shoujo style don't look so good to me, but she's the exception).