Definitely Messi. Have huge respect for CR7 as he had to grind really hard to get where he is now but Messi was born with a Godlike talent. He is undoubtedly the Greatest to have ever played for me.
messi influences the game in a deeper way. he's not just a great goalscorer/dribbler, but a great playmaker/passer - maybe the best, even when Xavi/Iniesta were around. almost all of the team's attacking play goes through him. and in his (rare) bad games, he's still a positive contributor on the pitch.
ronaldo is a great goalscorer but needs a solid team as a base to play off. for me, he's more of an add-on rather than an integral part of the team. take a 10 man team, add ronaldo, and bam, 1-2 more goals per game. i do feel like his floor is lower than messi, though. sometimes he has games where he's one of the worst players on the pitch (2016 CL Final?).
As a Barca fan I am definitely biased but I believe Messi is the greatest ever. He scores just as much as Ronaldo does, I say even more when it comes to goals per game for career, but he is also one of the best playmakers ever. If you want to go and say Ronaldo is the greatest goal scorer ever go ahead, I may not agree but I can see the argument. Messi has the goals and the playmaking so he can pretty much double the affect he has on the game unlike Ronaldo. For me its Messi.