The only person I ever remember actually seeing Fonz hit was Howard. (Howard was facing the other way and Fonzie thought he was a low life older guy who was dating Joanie). In Mother and Child Reunion, Fonzie asks a trucker to step outside, and when they do you don't actually see what happens but when the two come back in the trucker passes out cold and Fonzie's fine. It is implied that Fonzie hit him. Other than that, I can only remember Fonzie acting tough and the other guy usually backs down. Although during that fight in the gym (which was acutally Richie's fight where he asked Fonz not to step in) Fonzie did some weird flying kick to some guy's chest I believe.