Jayne, no doubt about it. Though he was kind of a comic relief figure for most part on FF I think AB played him so spot on, that it was very enjoyable to watch. "Sex, Muscles, Money, Thug = Jayne" is like AB describes his character with a lot of sympathies. He had a lot of potential to become an interesting characer. Would he have betrayed Mal in the end?
What I like about Jayne is that he always says what is on his mind and he is self-confident enough to give a damn about other peoples opinions. If he wants to wear a self knit hat from his mum, he does so 🙂 He is kind of Cordelia-like in this attributes.