I think that there are two candidates: Cristiano Ronaldo and Lionel Messi.
Of the two, Ronaldo has performed in a wider range of teams, often ones whose sums were less than that of the opposition. His national team is Portugal, always the underdogs on the big stage. He started at Sporting and was so impressive there that the greatest manager in the history of football saw the imminent need to sign him. Then he excelled in the Premier League, the toughest place of all. Then he went to Real Madrid and did better than anybody else has done there. Obviously, afterwards, he was a little bit past his peak, but he still tore it up in Seria A. Returned to the PL and, setting aside club politics, did well at an age that far surpasses the norms for retirement. Then he went to Arabia for the money, because why not. I don't give two shits about how he has done there because it counts for nothing. We'll afford Messi the same when it comes to the MSL.
Messi was fostered into a Barcelona team where everything was already set up for him to excel. There can certainly be no denying that he is/was amazing, but he didn't exactly have to buck the trend. It would have been the best team in the world without him, and it was even better with him. His unbelievable talent is not in question; but he didn't exactly put that team on his shoulders and carry them to success that they couldn't have achieved without him. Then he went to PSG; a club that always wins the league by default unless there's a once-a-generation upset. Wahey.
Then there's his national team; he often underperformed and was not really known to do well for Argentina until they finally won the WC. That seems to have wiped away all the many years before in which he didn't do very much for them. And it was a team that should have been amongst the giants, with or without him, and he only just got there at the last WC. This largely rewrote history, but the fact of the matter is that until they finally won it, Messi was known as a chronic underperformer for Argentina. They only won it when he was at the last gasp of his late peak. A stopped clock.
Compare that to Ronaldo who was, for all intents and purposes, Mr. Portugal. More than anyone, he has personified his national team. The only ones who can be said to have done him equal in that regard is Cruyff, and he had better teammates at his sides. When Portugal was in the European final and the opposition injuried him, Ronaldo took to the sidelines and animatedly guided his team to victory. Alright, it looked a little goofy at the time, but it clearly inspired their team to win. It wasn't just theatrics. You could tell that his aura was there even when he was limited to the technical area.
I posit that if Ronaldo had been Argentinean and Messi had been Portuguese; and if Ronaldo had been fostered from infancy at Barcelona while Messi had to make his bones through numerous difficult leagues, the tables would have been turned entirely. One is not better than the other; they are equal. However, one had to fight through difficulties and learn new leagues while the other was basically born into a team that was waiting for him all along.
We should also not forget that Messi was given permission to take growth hormones because he was a person of short stature. Not a dwarf, just commonly small. Barcelona negotiated permission for him to take substances that are, for other players, forbidden susbtances that constitute doping. This was justified by his small stature, but he wasn't physically handicapped. He was simply a bit small. He then had the advantage of using the same substance that other athletes use illegally to enhance their physiques in training, allowing them to spend many more hours on it. This is the reason it's a banned substance.
Through a medical loophole, Barcelona manage to concoct a situation where Lionel Messi qualified because he was 1,3 centimeters shorter than the average for his age when they signed him in his early teens. Then he was simply allowed to use doping. He doped, there isn't any debate about it. They just managed to acquire permission because he was marginally smaller than the norm for a 14 year old boy when they first tapped him up. This isn't some kind of conspiracy theory. It's in the public record. He fell slightly below some vague medical metric for age-based height, and they made some unknown Spanish doctor interpret that as approval to use what is very literally a doping subtance. The Barcelona way.
Nobody in their right mind can deny that Messi is an astonishing player, but he is a player whose talent was honed to this extent because they were able to get Spanish doctors (who? I don't know) to approve the application of a substance that is not normally allowed in sports. Let's entirely disregard the long and storied history that FC Bracelona has with doping and just look at the accomplishments of the two players. I think that had Ronaldo spent all his prime years at the golden age of Barcelona, and played for a national team that was an automatic favorite for the World Cup, he'd have done every bit as well; and had Messi been required to fight his way up through a third-rate league system, and make his bones in the PL, and then excel in Italy and all that, he would not have made the numbers that he did. That is my assessment. Take it or leave it.