Not a single punch in a fight, but a type of punch that a fighter used all throughout his career. My favourite punch of all time has to be Lennox Lewis's uppercut. Simply outstanding.
Hopkin's straight right hand Cotto's left hook to the body RJJ hooks (either hand) DLH's left hook to the top Tua's left hook Winky's jab JMM's uppercut
It was so good it had its own name, "the smash" :good Unfortunately he started to believe it was his excalibur and he tried setting it up every shot, and neglected and eventually lost his great boxing ability from his earlier career.
My favorite punch of all time is the leaping left hook/uppercut that Roy knocked Griffin out with. A thing of beauty.