Shannon Briggs always comes to mind. Knocked out more than one fighter with the first punch he threw. Accurate, started unbelievably fast, loved to counter punch and did it well. On top of all that, I think he might have the record for first round knockouts, slightly above Dempsey even. Also, Buddy Baer. Even bigger than his brother with comparable power and better boxing fundamentals. Also has a amazing amount of first round stoppages.