A lot depends exactly what you mean by greatest. Is it finest at their peak finest over a career, finest by statistics ,most aesthetically pleasing etc.
I think of it as being a form of football that captures the imagination appears to be far above the rest of the era and wins the games big prizes.
I first really followed football at the end of Pele's brilliant career , he certainly did all of the above.Maradona and Messi for me are actually very difficult to separate at their peak they were both just jaw droppingly brilliant.
Their eras were slightly different and Maradona had to cope with more brutal defenders and poor pitches than Messi, but Messi had to surpass a far higher standard of fellow footballer to excel than Maradona.
Personally I would give it to Messi on longevity grounds but wouldn't take serious issue with someone thinking Maradona edged it and don't really have the full knowledge to argue against Pele.
The one thing I can't take seriously is people who bring Ronaldo into the conversation.
