1. Ray Robinson KO Fullmer
2. Julian Jackson KO Terry Norris
3. JMM KO Manny Pacquiao
4. Manny Pacquiao KO Ricky Hatton
5. Tommy Hearns KO Roberto Duran
6. Rocky Marciano KO Joe Walcott
7. Julian Jackson KO Herol Graham
8. Antonio Tarver KO Roy Jones Jr.
9. Sergio Martinez KO Paul Williams
10. Ann Wolfe KO Vonda Ward

As of recent memory- yikes! Valdez vs Berchelt got me to thinking about best 1 punch KOs.
I had to put Ann in there because that shot was hard as a man's shot. She turned that shot over, brutal- Vonda made a hospital exit and was thankful to recover and regain the the title.