There are plenty of great options: Giggs, Scholes, Baresi, Maldini, Facchetti, Puyol, etc.
For me though, the greatest is Francesco Totti. The others above played for clubs who won multiple trophies, and all won the champions league at least once. Aside from a smattering of Coppa Italias and one Scudetto, Totti didn’t win anything with Rome.
For me, it’s easier for a top player to stay at a top club, rather than be a top player like Totti and stay at a club that was rarely considered the best in Serie A. For my money, he is the greatest one-club man of all time.
Who would you choose?
Totti is probably the greatest Italian player of all time but we won’t know because he stuck with a club several hundred classes beneath him.
Fun fact, he was sold twice - to Sampdoria in the late 90s and to Real Madrid in 2004ish, because the club needed money.
He refused both times, the second time the president thanked him because he had to do what’s right for the club but he didn’t want to sell him.