I've been thinking about this a bit lately, and here's what I've come up with:
Messi is clearly the better player and produces magical moments that leave you breathless (Nickoh is right, he could be the greatest ever) but Ronaldinho in his pomp gave you something different.
Raed brings up Zidane and Original Ronaldo with their instant momentum, and that's a fair point, but others have that kind of ability; Iniesta for example (and Messi is getting increasingly better from a standing start too). it's not what made Ronaldinho unique, it's not why everyone loved him.
it's nothing you can quantify or explain rationally, with logic or empirical data; it's a shared emotion felt between those who watched him play.
I wouldn't call him a Genius, but because that quantifies his greatness intellectually when his strength was spontaneous creativity. pure emotion expressed in magical movement. the end product almost didn't matter, if it came then good, but if not he was so brilliant to watch it didn't matter. it was football for football's sake.
in his prime, more than any footballer I've ever seen, Ronaldinho was an Artist.
Messi used to be an Artist, but the older he has gotten the more he has sacrificed that spontaneous creativity, that magical art, to become a footballer. end product is now the goal. he combines the two better than anybody since maradona; that's why he is the greatest player in the world, but the pragmatism of his pursuit of end product has diluted the purity of his art.
Messi will always be a better player, but that's not the point.