Opinions are like bellybuttons, everyone has one. Regarding which is the best granfondo my opinion is the "Maratona dles Dolomites". The registration period for the 2009 edition opens next month. You have to plan way in advance to enter this event (or get in through one of the authorized tour organizers such as Granfondo Cycling Tours, http://www.granfondoracingtours.com/).
Race date will be Sunday, July 5, 2009. Registration will be open from October 16th to November 6th, 2008. At this point you have entered the lottery. Drawings will take place in the second half of November.
See http://www.maratona.it/ for all details.
Photos: two images from the 2008 Maratona