I have thought Buffon to be the best, but its worrying he is not the best on crosses. However its still hard for me to think of a goalie better than Buffon.
As for the centre backs, I think Cannavaro when he's on form is the best in his position, but well, he's not on his best form. As for nesta, although he can be very good, IMO he is not worthy of a place in say an allstar eleven, as there are better centre backs than him IMO.
And as a team, they seem pretty hopeless together which is strange for two players of such quality...