Head = Jaw, chin, temple (Not recommended unless you really wanna hurt him).
Body = Lungs, Ribs, Stomach
I say go from Head to body, damage his head then drain him with body shots.
You already lost. In a street fight there's no such thing as "cheap." A real street fight has no rules. Kick, scratch, bite, elbow, do anything that's going to ensure you aren't the loser. Haven't you seen the youtube videos of the guy who gets knocked out then gets stomped on until he damn near dies? There are no rules in a street fight. Maybe you could start off with punching but if it gets brutal dude you need to do what you need to do. If somebody hit my girlfriend I wouldnt even fight him, I'd probably crack him behind the head with a bat, he'll never know what hit him. Or tell authorites lol. Either one works better then fighting him. Who fights fair anymore? It's 2013 bro.
Easiest reference point is a cross to the chin. But that aside, unless you have the experience and knowledge to throw power punches effectively and properly, you will not get a one-hit knockout. Then again the guy may be a punk, so it just might.
I guess here's a breakdown of good reference points
Cross - Chin/Bridge of nose/Throat
Uppercut - Chin/Solar Plexus/Kidneys
Hook - Temple/Jaw/Liver
Punch the guy who hit your girlfriend? what are you cinderella? buy a gun then shoot the guy in the head....That guy deserves more than just a punch...
Stay calm, cover your self at all times, don't swing big, take smart shots. Stay on your toes. Go watch some videos on YouTube.
corner him and start unloading but make sure that your punches actually cause pain.
So im about to fight this guy who hit my girlfriend , i need to know where i should punch that would hurt the most or where i could puch for a one hit knock out ? Also i will not punch in the nuts , thats cheap