All the 1/6th scale Obitsu's come headless, if I'm not mistaken. All the Obitsu dolls come with several neck pieces for different sized neck holes, so what head to get depends on if your after a more realistic-sized head, or a cutesie over-sized head. Just make sure that you get a match to your doll's skin-tone.
Check out the Junkyspot's 27cm heads for a more realistic sized head. I picked up a set of the W-01 heads, and I think they're on the small side for the 27cm body (which is why April ended up with our 27cm hard-bust body instead of Riruka
). They would probably look more in scale on the smaller 25cm body (Though I don't have a 25 to check to be sure.) I'm not a huge fan of the different knees on the 25, and I'm not completely sold on the soft torsos (Though I love the Megahouse doll I have on one). There are also a few different eye hole heads that fit the 25/27 bodies.
If your after the more cutesie look, the 23cm "child" heads, like the Parabox Neko, would work, I think. I don't know much about them, though.