The request is for the "best", measured by "effectiveness". The biggest may not be the best. Shavers belongs in any mention of "biggest" but he's short of "best" for example.
Both Louis’ hands, the power they carried and the variety of technically sound punches he threw with them made it likely that he would rate highly across the board and would top the charts in at least one of the punch type categories. Louis could throw beautiful straight right hands and he also utilised a kind of nuanced overhand right, which rose slightly initially with the elbow out a little bit more. Sans the same power as Louis, Ali and Holmes had terrific, laser fast and accurate rights of somewhat similar quality but I think I would rate Ali’s as better.
Based on pure facts, it is got to be Joe Louis and no one really comes close imo, the evidence is right there.
But his competition was weak compared to what Lewis, Foreman and Wlad had to face. Those cats would have walked through half of Louis' resume in an afternoon.
What Ali's doing with his right hand leads, right cross and a chopping downwards right to the chest area pre-exile was genuinely exciting. His right hand is underrated and capable of changing a fight with a single well-planted punch. George Foreman's chin certainly would attest to just how sharp and snipey it was and it was the right hand at the end of a combination that made Sonny stumble in the first fight. Joe Louis' right has no competitor in the division and after that, I cannot see an argument outside of Lewis.
Louis was a perfect puncher. Smooth as all hell. I have yet to see a heavyweight as smooth as him/with that punch technique. Sure, his time came and went, but he was it.
Which is why I'm going to make a pick of Larry Holmes. Because when you're not saying if it's a straight, or a cross or whether it's the strongest and that's why it's effective, but in terms of overall effectiveness throughout the course of a fight, for any reason, I think the ability of one mister Larwold J. Holmes has to use his right hand during the course of a fight is arguably the best and if anyone says otherwise, I spit on the names of their ancestors and call their children uncivil. I make the rules here.