On the basis of ESTPs I've know, I'd say ESFP is more likely. The Fonz wants to ingratiate himself and have a good time; an ESTP would want to manipulate the situation to entertain him. Less Barnum; more Elvis.
Actually, I guess there you go. Elvis has been described as ESFP (which seems right to me), and that's a pretty close analog to the Fonz.