Even the most iron chinned have their off moments. Plus, kd vs slip is not as clear cut as people make it out. Sometimes you connect but the person is going down from their own mismotion, or they are hurt but go down from a push etc. I made a list of debatable, uncalled knockdowns for some of the greatest chins in HW history who were never officially knocked down. Please add to the list if you can think of any. Oliver McCall: For many the standard of chin in boxing. He was fairly clearly knocked down by Henry Akinwande in round 4 or 5 of their fight though, but disputable enough to be called a slip. Vitali Klitschko: Round one vs Corrie Sanders. Personally, I think if you look at it closely, Vitali had already started to recover from the damaging blow, and Corrie got frustrated and pushed/tripped him. It was a valid MMA takedown but not a valid boxing takedown imo, correctly ruled a slip. George Chuvalo: 2 uncredited kd's vs Bonavena, close enough to be called slips.
I think he's clearly an all time great but you're right on the first two. Read the original post too fast and skipped over the heavyweight requirement.
The most interesting was Akinwande - McCall as Henry isn't even a puncher. Oliver was caught off balance, he was never hurt, but if not the ropes, he would've been officially knocked down by a punch. That's the guy who was never dropped by best punchers in the game.
The punch at the final bell for Tua vs Rahman. Tua was undoubtedly off balance but it could have been called a KD had the punch landed 1 or 2 seconds prior. It was as the bell sounded so tough call for the Ref.
