That's a tough one. I'm partial to pythons and boas. I guess it would have to be Amazon Tree Boas.
They are small, thin, have big wedge heads that look venomous* and come in such a beautiful array of colors.
*closest I'll ever get to keeping venomous
They are bitey and inquisitive and I just love them, little devil shoestrings that they are.

BRBs are a great choice and were one of my favorites when I started out and saw one in a book. Here's a photo of the one I used to have, he was a great snake: