From a technical standpoint Messi is probably better.
But what makes Maradona the best football player in the world is that he transcended football. His football changed things. Brought pride to both Naples and Argentina more than a football trophy could've.
The difference is that Messi so far failed to deliver a world cup. Just didn't happen. Maradona played a minor role in bringing home the 1978 world cup and took the lead role in closing the deal on the 1986 one. Messi has so far not delivered on that front.
World cup is a different beast from the Champions league. The CL takes place over a much longer period of time and teams can better recover physically and emotionally. The world cup is a tour the force that Messi was never able to dominate and win. Messi is technically superior, no doubt, but was never able to consistently shine under psychological pressure. Delivering under pressure is what the likes of Maradona and CR7 have been able to do and that's what makes many soccer lovers call them the "best".
Maradona had more charisma and leadership though.