There are a lot of articles out there about what's legal and what's not, how about one for "what's worth it"?
I am well aware that violence should never be the answer. I also know that you should never punch anyone unless they've committed to violence first. Still, occasionally, some of us come across someone who needs to be punched in the face.
Before we get to the legalities, let me say only on the rarest of occasions will you be glad you punched someone in the face for more than a fleeting moment. You will most likely be ashamed of your actions and possibly suffer a broken hand (because faces are hard). You will also go the rest of your life worrying whether that person is going to come back for revenge in some form.
Then there's the fact that if you punch somebody you are at the very least going to end up responsible for their medical bills. The better face-punch you delivered, the better chance that person is going to need immediate treatment and possibly facial reconstruction (which, as they sound, are very expensive). You'll probably end up paying for their pain, suffering, and time off at work as well.
So if you think your wallet can handle the repercussions of punching someone in the face, you're still looking at a minimum of up to one year in jail and a fine of $4000. Punching someone in the face definitely constitutes at least a class A misdemeanor of "assault with bodily injury". Depending on circumstances, that charge can easily ramp up to felony charges, and depending on the circumstances get you life in prison.
Whether in real life or in the movies, we've all seen one guy stand down and say, "you aren't worth it". That is a very real moment because punching someone in the face won't necessarily ruin their life, but it can ruin yours.