I can only comment on players Ive watched since the early 90s.
I don't remember much of Baggio at Juve, I was too young. Never seen Platini play at all.
So Del Piero is the greatest for me.
If you wanted to choose someone for being the best player on the world stage overall not just for Juve then it's probably Christiano Ronaldo, Baggio, Platini or Zidane... Buffon is up there but we shouldn't compare goal keepers to outfield players.
But the greatest ever Juventus player due to history, loyalty, achievements and other things on and off the field then Del Piero is the greatest Juventus player ever.