Hmm I totally forgot what was my answer the last time I answered this question (or something similar). I think I said Sayu and Eripon the last time (for writing, at least).
Reading-wise... let's see. I haven't been able to get into reading anything but stories about the babies (the new kids, ha) recently, and since I'm the one writing them most of the time, I have to say that when I read Japanese fics, they seem to love writing fic about Eripon or Riho, but I have a personal fondness for panicky Ayumi (it's like, a universal standard, I love bullying Ayumi in all my fics and so do the Japanese fans lmao) in Japanese fic.
Riho's good too, if well written, but sometimes you get really weird characterizations that make you go WTF? when you try to read them. I love all the babies, so any fic about them is welcome, honestly! (Well, qualification of well written). And Eripon is too amazing to get wrong...I don't think I've read bad Eripon fic so far...shocking Eripon fic, yes. Bad Eripon fic, no.
Cos Eripon happens, yo!
Writing-wise... My oshi is Riho, but writing her is like trying to tear my fingernails off one at a time sometimes =_=. Eripon is always fun to write, and I think I have a thing for evil Fukuhime o_o;;; Well, a Fukuhime who doesn't really conform to ordinary morality at least
Her ojou-samaness does tend to warp her perceptions a little...haha. I love Maachan to bits and when hitting impasses, a well applied Maachan will break even the most awkward of situations.
I love all the children and love writing them all, so I can't really pick a favourite! They're all amazing! Haha.