I was gonna vote Komachi because Touhou. But then I stopped and thought about it, I haven't read Bleach since the "Aizen's a Cheater" arc ended, but I recall that Rukia actually didn't suck that much, plus Komachi's pretty danged lazy.
I went with Komachi in the end anyways 'cause I figure everyone in Gensokyo is a master of dodging EVERYTHING, and that without spellcard rules, the major boss characters are probably a lot stronger than they are in the games. ^.^