If you asked me about five years ago, it would've been Messi without question. He was sharper, more skillful, and a cleverer player than Ronaldo, who by contrast seemed to sort of "brute" force the issue. A bit like the contrast between Federer and Nadal in the tennis.
However Ronaldo, since then, at 33 ffs, seems to just be getting better and better and. As someone alluded to above, he probably could play until he was 41!
He's had to adapt his game a tad given his former "lightning" pace is now only merely "very quick" pace, and has *somewhat* been able to carry teams that he plays in the glory.
Barca over the last few seasons with Messi, Neymar, Suarez & Iniesta etc for my money didn't rely on any one of them. Ronaldo, despite the likes of Bale, Rodriguez, Kroos, Benzema etc, has still been the rock on which Real was built on to win tournaments - which is probably why they're a better cup team than league side.
Similar is true of internationals, where Ronaldo seems to be able to drag the very very best out of an ok Portugal side, whereas Messi doesn't get the same out of Argentina.
In short, for me the best player in the world pretty much lines up with their Ballon D'Or wins.
2009, 2010, 2011, 2012, 2015 - Messi
2008, 2013, 2014, 2016, 2017 - Ronaldo