Maradona's still the greatest for me.
It's a fun, pointless argument and there is no definitive answer, mainly because if you ask 100 people to define what makes the greatest you'll get 100 different answers. For some, being the best at club level is enough. That's a fine and valid position to take. For me though I'd expect the Greatest of All Time to perform at that level at both club and international level.
A lot of people on here denigrate international football and World Cups. "It's not at the level of Champions League" for example. Which makes you wonder, if it's not at the level of Champions League, why isn't Messi performing as well, if not much better, in an easier format? It's not for lack of effort - it means more to him than performing for Barcelona. It's because performing at international level requires almost a different skillset. Club football may have the edge techinically and organisationally, but the mental pressure of performing in the World Cup knock out stages is like nothing else, and some players handle that better than others.
Don't get me wrong, Messi's always been 'good' in those WC knock out games, but if you see him for Barca and then see him for Argentina in the business end of a tournament, you can't tell me he's not much worse in the latter games. His level dips significantly. He goes from being godlike for Barca to just 'good' for Argentina. I don't think it's too much for the GOAT to maintain his excellent club level during the high pressure international matches. The other greats managed to, such as Pele, Maradona, Zidane, Platini, Ronaldo etc, so it's not outlandish to expect that from the GOAT.
I don't even think he needs to win the whole thing. He could just turn up and stamp his authority and class on a tournament like Baggio in 94 and Zidane in 06. That would be enough for me anyway to know he's got the mentality and the cahoonas that I think the GOAT should have. Each to their own though.
As an aside, it makes for fascinating reading some of the mental gymnastics you Messi obsessives are coming up with justify him not yet turning up to a big international tournament. Seriously fascinating.