Lionel Messi.
He is the best player in the world and arguably one of the all time greats already.
We'll start off with some pretty simple stuff.
Ronaldo: 31 starts. 1 sub appearance. 32 goals, 9 assists.
Messi: 27 starts, 5 sub appearances. 37 goals, 18 assists.
Now for me though, the reason why Messi is the best and the reason why it hasn't worked for Argentina is because for Barcelona, he is just another player. He is better than everyone else but he has a role in a team, he plays for the team. You watch in hope of him getting the ball and dribbling every time, wanting him to take players on, does he? no. He plays it simple, he doesn't go searching for the ball, he plays his role, waits and comes alive at the right times. Barcelona do not rely on Messi and that's why he's so good because Barca don't just look to Messi, they look to everyone in the their team which means when teams play against them, you can't double/triple on players because they will exploit other gaps. Pedro, Villa world class players, Iniesta, Alves, Maxwell, Xavi, all heavily support the front line. Teams try to stop Messi but that doesn't stop Barcelona which is what makes them a great team and also makes Messi a great player. He understands it. He draws players into him and then releases it. I know perhaps people think...this is more talking about Barcelona as a team rather than why Messi is great but it says a lot about a player who has so many goals and assists and yet is so much of a team player. He knows the right times of being a team player which means he'll be part of great Barcelona teams and also when to be an individual, hence why he's such a great individual. He has the perfect balance.
Where as for Argentina, Maradonna tried to make him the star, the one man team. He isn't that player. That doesn't make him not as good but Messi is a forward player who is going to be in/out the game but then when he's in it, utterly unstoppable. He's not the sort of player who is in a game all the time who starts the moves off. Maradonna wanted Messi to be Maradonna but Messi is Messi, they are different players. He's not used to that role, it's hard for someone to completely adapt from being a team player to almost being forced to being an individual of a team.
Messi has pace, his dribbling ability is not matched by a single player. His creativity is up there as good as anyone's in the world, his passing in the final third is destructive, his vision and footballing brain as well as team ethic and work rate is top notch. His strength and power and balance is so under-rated, it's almost impossible to knock him off the ball.
Ronaldo scored 42 in 07-08, 9 assists in 49 games. During that season, Man City weren't the same team as they are now. Liverpool were better but still not title challenging, Arsenal were a very good team but not great, similar to now I'd say. Chelsea under Grant weren't a great team although still very good. If Messi was playing in that season from the right and through the middle at the level he is playing now but back then....I'd say he'd have similar if not more. He doesn't have to prove anything for me, why would he leave the best team in the world? The reason Messi has nothing to prove is because the last two seasons, he's been clearly the best player on the planet. Yes of course, if he moved, he wouldn't score and create so many, nor would Ronaldo. Ronaldo before moving to Real scored 26 in 53 the season before, so I mean, it's not as if he was scoring at the same rate.
Ronaldo is a world class player but for me he's just a level behind Messi, maybe not in terms of talent with the ball but his mental attributes of knowing how to play for the team and being that team player which is massive in big games, a reason why Barca destroyed Madrid. Also I think Messi is a far more creative player and has a better all round game. Messi can score goals like Ronaldo but he can also pass a ball like his team-mates, where as Ronaldo for Barcelona, he'd probably have 25+ goals, but I would also believe that firstly Barcelona would be a worse and less successful team and also Ronaldo would create far less for his team-mates also. He just wouldn't fit into a Barcelona style. He doesn't have the mentality.
Ronaldo has more attributes in terms of ways to score goals, left foot, right foot, head...but Messi is for me just the all round better player in individual and team aspects.