It's the question no-one in the world can seem to agree on, a little bit like that whole smooth vs chunky peanut butter thing (I'm on the side of chunky by the way).
The problem people have in deciding, is simply because they are both outstanding players. One of a kind - except there are two of them.
Both have brought glory to their clubs, both have the ability to "carry" the rest of the team and make the difference between winning or losing.
Both score plenty of goals, terrorize defenders and get fans jumping out of their seats with excitement every time the ball touches their feet.
So, the only way I think it's fair to compare them, is if we let the numbers speak for themselves (there's actually a whole website dedicated to comparing the two: http://messivsronaldo.net/)
Ronaldo vs Messi stats
If we look at goals in all competitions, this is how they stack up in the past few years:
2013 (so far) 11 vs 15
2012-2013 60 vs 55
2011-2012 73 vs 60
2010-2011 53 vs 53
2009-2010 47 vs 33
So based on those goal stats, it seems like Messi is actually quite a way ahead of Ronaldo (and the assists figure tells a similar story).
But it all becomes crystal clear when you look at the honours that each player has managed to win as part of a team. While Ronaldo is an amazing talent playing for one of the best teams in the world, Messi has helped Barcelona dominate every footballing competition they have been eligible for in the past few years.
Here's what their respective trophy cabinets look like:
The site mentioned above goes on to analyse and rate every aspect of their game (fascinating read if you're interested), but for me the evidence is clear - Messi edges this battle!