I voted for Buster Douglas.

12 of the 20 are before 1969; a bit too broad a question. It's like asking what was the best home run of all time.

There are some knockouts that are stunners (Corrales / Castillo), there are some that are the righteous gratification at the climax of a great performance (Ali / Foreman) and there are some that remind to grab the chips and dip before the first bell (De Leon - Sod L.)