Personally, I don't think he is "THE" greatest but is certainly up there with the absolute best.
The reason I don't say he's the absolute best is fairly simple - he's never really performed in the World Cup apart from this one. He's also only been world class at one club, Barcelona, who I regard as the greatest club side of all time whilst he was there. He was the best player in the best club side.
I saw Maradona at his peak and he pretty much single-handedly dragged 'average' sides to greatness. Argentina weren't the best national side in 1986, but Maradona was the best player on the planet. That's how good he was. That game v England almost defines he to us English and we saw both sides to him. However, he burned out and didn't have the longevity of Messi.
We will never be able to compare between Pele, Maradona and those from other eras. However, we can compare Messi to someone in his era. I know it's not fashionable to say this, but Messi and Ronaldo are the two greats from the last 20 years, easily. Both were on a different level to anyone else. Ronaldo at Real Madrid and Messi at Barcelona were players who turned any game in their favour, it's as simple as that.
However, Ronaldo has proven himself in England, Spain and Italy. He proved he could do it in other leagues and although Messi is past his best, he's not really lit up a poor French league has he? As I said, his performances in previous World Cups have been average and nowhere near the level he can play. This isn't to denigrate him, but the question is: has he done it outside Barclona?
I consider myself lucky to have seen Messi and Ronaldo play live and seen Maradona on TV. All 3 were capable of utter genius. However, I've just heard too many people who saw Maradona, Messi and Ronaldo play say that Pele was the greatest footballer of all time.