Err...
This is a little hard. ><
But I'd probably say Mion. She's outgoing, fun, loud, competitive, and sort of deranged. (coughWatanagashi-hencough) I love her character. And it doesn't hurt that her seiyuu is Yukino Satsuki. (Yoruichi!!
)
I didn't really like Rena at first because she seemed like a bit of an airhead... but then when she switched to her Oyashiro-sama mode, I realized there was more to her than her cute "kana~kana"... in fact, the "kana~" started to sound creepy after a while.
Oh, and I have to say that Rika is awesome too. She seems innocent and caring, but also ambiguously mysterious at the same time... is there more to her? Most likely. Seeing as the last three arcs have covered each of the girls respectively, the next arc will probably focus on Rika.
Also, props to Keiichi for being an unconventional harem lead. He's crazy enough to kill people.
Ahh! I'm starting to get Higurashi deprival! (Waits patiently for next episode...)
~ Noodlehead