Messi scored 91 goals in a calendar year at his peak. Nothing R9 or Ronaldinho did in their careers comes close to that.
Anyway, Cristiano is very close to over taking Messi in my opinion, for longevity and what they have achieved, but peak Messi was something else entirely. The best player ever at any players peak, but peak alone doesn’t determine the greatest. In that case, Pele and Maradona are 1 and 2 in that order for me.
This is the exact "mental gymnastics" that I am referring to. I only referenced R9 or Ronaldinho in terms of talent/ability, but you have conveniently shifted the goal posts to "goals scored in a single season." If you want to play that game, you will 100% lose. You want to know why?
Because Pele scored 127 goals in a single season. By that metric, Messi CANNOT be the GOAT. He is off by over 30 goals. If you want to argue that "well times are different now," well guess what? Yes, they sure are. Back in Pele's day, you did not have single teams monopolizing the top-talent that allowed players like Messi to score for fun.
Do I care that Messi scored 4 goals against CA Osasuna or a hat-trick against Almeria? Not really. There is a legitimate argument to be made - why aren't other La Liga players scoring so much against such cannon-fodder? Well the answer is simple. Barca was the best team in Spain by a mile, and Messi was the best player in La Liga at that time. Duh, obvious.
But agreeing that Messi was the best player on the best team (by far) for a few years is completely different from concluding that this makes Messi the GOAT. We need some objective, logical standard.
And as mentioned before, every objective, logical standard falls against Messi. Unless you do mental gymnastics and believe that the GOAT-criteria is "most goals scored in club competitions from 2006-2013."
Here are some other criteria:
WC goals scored - Messi falls flat on his face
Goals scored playing for other teams - Messi never ventured outside of Barca
International competitions won - Messi loses
The only statistics that make Messi look good (and he was undeniably the best player in the world for several years), are club-centric. Messi only enters the discussion for GOAT if you turn a blind-eye to every other metric and disregard achievements of players like Pele.