You're right that Frazier was a clean fighter. I don't recall him ever using elbows and on the rare occasions he hit low it was accidental. Louis said he hit Schemling low a few times in the first fight but this was after he blacked out - he didn't remember anything after the fourth round.
Riddick Bowe was filthy dirty...he kicked one opponent and was hitting on breaks and such against Holyfield...Ali poked Ernie Terrell in the eye repeatedly. (According to Terrell, at least). Some of Holyfield's headbutts were intentional and some weren't. For example, there were plenty intentional butts in the first Tyson fight and he deliberately tried butting Lewis in their first fight. But Holyfield would dip low for leverage before unloading power shots and clashed heads with Tyson in the rematch. I think the "bite fight" butts were accidental.
Comeback Foreman fought clean, though he may have scored the rare accidental low blow. Chris Byrd fought clean but sometimes had to be warned for punching with his hands open. David Tua was pretty clean too overall - though like most swarmer types he hit the occasional low blow or elbow.
Now that I think about it, there is indeed a shortage of heavyweights who didn't cheat.
I noticed Golota absent from the line up, I'm sure them low blows hurt but were they deemed intentional or just plain impulse. His trainer was so dumbfounded by the fact his man was winning both Bowe fights only to throw them away by disqualification. I'm certain some of Holyfields headbutts were strategically disguised as unintentional but were a tad dirty but sure did not warrant Tyson's crazy bite retaliation.
Ali's pulling grabbing against Frazier & Foreman seemed almost a form of tactical gamesmanship but to be fair what could he do against 2 attacking hulks, it seemed like his only survival option when he had lost his foot speed & never could contain Joe's left hook by learning to parry & guard against it, unfathomable from a man of his ability.
2 men with similar surnames get my fair fighter vote. Lennox Lewis was pretty fair for a modern heavy & certainly Joe Louis from the past but I'm sure there are many more of The Good, The Bad & The Ugly but they have been covered by other commenters