Del Piero without a doubt. Besides being the all time leading goal scorer with 197 goals (and counting), he has lead Juventus to 7 scudetto, 4 Italian super cups, 1 Coppa Italia, 1 CL (lead them to the final also in 97/98, where he was the CL top scorer, and in 02/03), 1 European Supercup and the Intercontinental cup (where scored the winning goal).
Not to mention he has habbit of scoring when u need him the most. Countless examples where he has scored in big games to help Juve secure titles. Super goal vs Fiorentina (which he has called his favourite ever scored): (copy and paste into address bar)
Code:
http://www.juventusmania.net/lpe427/ale_viola.avi
Another example would be Del Piero scoring a hattrick on his 1st ever start, vs Parma. Many more examples.
To cap it all off, unlike former Juve greats, he has won the WC.