I would have to say that my favorite character is Mirco, a slave in Witherwilds. For some reason, his is the most comfortable, facile POV I think I've ever written. His worldview is pretty dark and selish, but he is a product of his hard life. His voice is churlish and stark at times. But, whether he wants to admit it or not, he's capable of caring for others.
Least favorite....hmmmm. I can't think of a character in particular that I *don't* like to write. However, there are times when I have a problem catching the essence of the character--not just their voice but their motivations, fears, etc--and writing them becomes a chore. I know I'm failing them, and I hate that.