Xavi was a king at Euro 2008, arguably the best player in that tournament, he was amazing at WC 2010 as well.
Of course Pirlo too was majestic at the 2006 WC, no doubt about that, the period between 2004-2007 he was world class for Milan too, but Xavi has been more consistent and despite not being as complete a player as Pirlo is, he does what he's good at better. Xavi for me has been the best midfielder in the world for a while now.