Shavers, Foreman, Liston, Dempsey(There must be some reason he still has the most 1st round kayos).
I don't count wilder because of todays opposition, I know he's fighter du jour for this vote but I don't believe it.
If you've been watching the sport as long as I have, you've seen guys like Milton McCrory who were kayoing everyone until they stepped up in comp and they became average punchers. We just don't have a good level of comp today to measure any of these heavyweights by. I know it'll stir up shit, if I have time I'll respond.
Edit: Tyson is another great example, he was kayoing everyone until he took his first steps up, then, he was at least extended by Jesse Ferguson and almost beaten by Quick Tillis, then extended bay Jose Ribalto, and Bonecrusher Smith and Mitch Green the latter two never even being hurt on their ways to decision losses.. The class of the opponent matters about 50 percent as much as the one doing the kayoing.