Larry Holmes isn't criticized for fighting in a weak era just because he didn't fight in Ali's era but he also had close decisions [also against Norton].
Rocky Marciano isn't criticized about cherry picking. He fought everyone in his era, but as you said; he is criticized for fighting in a weak era where many of the champions were old men.
Joe Louis was criticized for getting for his decision win over Walcott which i felt he lost, but on the remach Louis left no doubt on who the king was. With that said; he is also criticized for fighting in a weak era.
Every single fighter was criticized for some reason or another.
If you fight allot you're bound to fight someone who gives you trouble. Henry Armstrong was criticized for having problems and controversial wins against some top Mexican featherweights. Floyd Mayweather is criticized for some wins, especially against Castillo.
And if you fight very rarely you're criticized of ''pick & choose''. Sugar Ray Leonard fought the best always but his inactivity led some to speculate that he dodged Arron Pryor even thought Pryor never fought at 147lbs!
The closest thing to an ''inarguable record'' that i can find is James Jeffries.
He fought 100 years ago but he fought absolutely the best of his time including black fighters [a major stain in many heavyweights back in the day including Jack Dempsey & Jack Johnson].
Jeffries had won most of his major fights by knock out and his only 2 draws were fights where most spectators thought he clearly won.
His only loss came after a 6 year retirement.
James Jeffries only has 23 pro fights but he won the undisputed title by his 13th pro fight and was very active heavyweight for his day fighting around 3x per year
Note on Larry Holmes's argument. Ali happened to fight in the greatest era of HW boxing where in his record he beat Liston, Frazier, Patterson and Forman. Along with Ali these are 5 of the top 20 HWs ever, this made people think that that was the standard and every HW champ after that era was accused of beating weak opposition like Holmes, Tyson, Lewis and the K Bros when in reality there fighting the standard depth while Ali's era was the exception.
Back to ur question, my pick would be Carlos Monzon, Pernell Whitaker, Vicente Saldivar and Henry Armstrong as having inarguable records
Ali won several fights late in his career that he should not have won.
this does not actually reduce his career since he was a great champion that fought all the contenders and did not duck people.
Holmes and Marciano both suffer from fighting in an era where their competition is not considered to be as good as the Ali era.
Comparing greats from different times is always a personal judgement.
I am still going with Louis.
Andre Ward
Andre Ward because he's cleaned out the super middleweight division and has never won a controversial decision ,he just doesn't get the respect he deserves.
Lennox Lewis, he fought great competition, and avenged his two losses.
People argue about Ali's record because of decisions, Marciano's record because of cherry picking and Larry Holmes for not having the best era is there anyone who is inarguable great?
