Not sure how well it would work, but I always like the idea of a ducking roundhouse (ducking left roundhouse to chest). This is number 6 in our kicking one-steps.
Look at them with a very serious look and say "Don't I go to church with your mother?" Half of the time they will turn and run if they don't it will give you a good half of a second to set your next move up.
i would back up and get into a well balanced defensive stance with plenty of room between us so i could size him up, i would maintain full eye contacted.
I would wait and watch to see when the moment of cognition in his brain happens. (that's when you read the opponent and you can actually tell the moment they think of what their next move is.)
then i would intercept their attack (which is a great way to rattle your opponent) and proceed with some nice exploitation of their weakened position, IE. counterattacks (if they are still trying to get off some strikes), combos (if you are feeling like delivering some punishment.), or just finding holes in there defense for some trapping or a take-down.
but to be honest anything could happen in a fight but the answer to the question, that would be my first killer move.
then again i might just break their jaw if given the opportunity.