I've been working on a few non-human personal projects for a while now. A dragon that came into being and now another who's fighting me every step of the way. I don't know what your favorite part of sculpting is, but mine is that stage when you're finally able to get them to stand on their own. There is this great sense of accomplishment when the joints and legs finally come together to make the critter stand. I'm at that part now, a little rushed since my own ankle joints snapped off and I'm in the process of redoing them. Yet, I was curious what other thoughts were on the matter?
Personally I love the moment when the BJD is first strung, and you get to see if the parts all work together and whatnot. Least favorite moment is probably sanding/nitpicky refinement.
Most favorite is finishing the head, and having the doll really come to life. Least favorite is making joints! I suck at that!
Defiinitely the "coming-to-life" moment, when the parts of the face stop being separate parts, and seem to come together to create something with a personality of its own. Least favorite... when I make some change to the face that undoes that effect. ;P
My least favorite part isn't necessarily part of the sculpting process, but because I have no real place to work, setting everything up and then putting it all away each time I want to work on something can be a chore, especially if I only want to work on something small for a couple of minutes. I haven't gotten to the stringing my doll up part, but I can imagine that can be very rewarding.