Tsumugi! Even though Mio plays bass guitar, the best instrument in the entire world, she's a little too.. Moe. Plus Tsumugi is... I dunno, awesome x10.
Why's Mio so popular (reminds himself why he voted for Mio) ... tall, shy, long hair maybe, non-boisterous ... kinda reminds me of a less bustier version of Shizuoka from Umisho.