Suing the guy who threw the punch, or his buddy the bus driver would be a complete waste of time. I doubt you'd get a lawyer willing to take the case. From the description of the incident, it's unlikely that either if these two have any assets. Suing the bus company would be a different story. At least they have liability insurance.
The best bet for satisfaction wit regard to the two miscreants is to get the perp arrested and the bus driver fired.