Off the top of my head:
1. Frazier's left hook against Ali in Round 15 of their first fight. This is the hardest, cleanest punch I have ever seen... and Ali was back up right away. Tells you something about Ali's chin and heart.
2. Tyson's right hand KO punch against Michael Jack Johnson.
3. Hearns' right hand that KO'd Duran.

Honorable mention: Julian Jackson's straight right that destroyed Terry Norris.