Ronaldinho, without a doubt.
Ronaldinho can play as playmaker, attacking midfielder and supporting striker, so can Kaka. that means they can constantly shift their position through the game. I mean something like this:
----Ronaldinho--------Ronaldo------
-----------------Kaka-----------------
or:
-------Ronaldo-------Kaka-----------
--------------Ronaldinho-------------
or:
-----------------Ronaldo-------------
-----Ronaldinho-------Kaka---------
That means: say goodbye to man-marking (coz it's impossible for a team to man-mark both Kaka and Ronaldinho), say goodbye to all these tirednesses from being the key player and its negative aftereffects, say goodbye to key player injury worries, etc.
They would be simply unstoppable... Like our past decade Dutch triangle, this time a Brazilian one.
BUT... I don't think we're gonna get him!