For me Messi. Although CR7 can score pretty much any type of goal known to man, Messi's overall skillset and contribution to the team is more valuable. Generally you see that in the rare games where Ronaldo doesn't score he's not very useful in the way of assists or setting anything up. Whereas Messi can be just as useful when he doesnt score. Position wise Messi's skillset makes him more versatile than CR7, Ronaldo can play on the left or as a striker. Messi can play RW, Striker or even as an Attacking midfielder(i know he doesnt play here im saying that he could).
To sum it up for me. If an extremely talented player comes along and works VERY hard. He could be the next Ronaldo.
But there will never be another Messi