Maradona, for me.
It's reasonable to say Maradona and Messi are directly comparible in ability, size, and style, so we could make a case to say, from a base-point, they're about as equal as it gets.
Then, take away all the advantages Messi has had: sport science, heavy data analysis, higher quality teammates (club/national), better pitches, and so on.
Take these things away and, suddenly, it should all become clearer.
Messi is still the 2nd best though... and I don't even like the man.