I really like super thin Billie. Of course, that doesn't mean I don't love him with a little more meat on his bones too. He was not exactly a beanpole during early 21CB (he got pretty thin for Broadway later on) and that's one of my favorite looks for him. I think the ideal is a little on the thinner side, but still moderately curvy. Like this:
He's not ultra skinny, but you can tell he's strong and fit. (Of course I used 21CB Billie as an example. He's so prettyyyyyyy!!)