Without taking anything away from the other girls, for me it's Tomo too.
She handles her instrument best among all the girls, sings well (while simultaneously playing), writes more than half of the bands lyrics. Technically she's superb. Over the last year, I can count only on one hand the mistakes she makes while playing live.
I recall a translated interview where Rina I think it was, who said that Tomo is really good and can learn really quickly, so much so that it makes the other jealous
This is one reason why I like Tomo so much. It's the cognitive dissonance. The first impression is all wrong. She looks and sounds like a bimbo what with the chipmunk voice and the genki girl behaviour and the goofing off and the pranks and misadventures etc but when the music comes on...
Rina is a close second and catching up fast. Flawless drumming, doesn't lose tempo (try matching their live perfomances with their recorded material and you will notice the tempo is spot on everytime, I have actually done this many times), can do backing vocals at the same time. Remarkable for someone who nearly quit the band for "not being good enough".
Her lyrics are surprisingly deep for a young, giggly girl and she's also writes music now too.