I think Pep and Whitaker are the top 2.
I put Whitaker #1 because he moved up thru multiple weight classes, whereas Pep was a featherweight for about his entire career.
Also, Pep was KO'd several times (even in his prime). Whitaker never was.

Whitaker had plenty of fights against world champs where he didn't even have a scratch on him at the end.