It's great to see Ronaldo back at Manchester United. And I imagine those who are very big on nostalgia and take a sentimental view on things will be overjoyed.
I can actually buy into a bit of that, as I prefer watching Ronaldo now in his latter days. Although for me, at least in terms of footballing talent wise - he's not even in the argument with regards to the best player ever. Athleticism is a massive part of his game, as well as consistency, but he doesn't have the pure skill or on field intelligence when compared to some of the greats. Take his dribbling for example, it's often very crude. He obviously has good technique, but it simply can't compare to the greats. But he is an extremely consistent athlete, probably more so than the greats. But it is not that, that generally makes one great.
Messi is definitely up there with the greats. Like Ronaldo, he shows consistency that isn't really seen. Although he doesn't have the athleticism of Ronaldo, he can do most other things better. Some a lot better. Which is why he has a similar scoring record to Ronaldo, but a far superior record with regards to assisting his teammates.
Is Messi as good as Maradona was? Probably not. Although of course he's been more consistent throughout his career. It's doubtful that Messi could have done what Maradona did with Napoli or perhaps even Argentina. At their very best, Maradona was just a bit better. But we should be aware that Messi's career is still not finished.