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?
There will be a bias towards more recent players but there can’t be many more compelling cases for the ultimate one club man accolade than Nat Lofthouse -
Born in Bolton, joined them at 14, first team debut at 16, & played for 19 years. retired at 35 through injury & became a coach the next season… after 7 years working his way up he managed them, then became chief scout, admin manager, executive manager, then caretaker managed them aged 60. finally he became club president. Died in Bolton too.
Could have gone anywhere else in his career, would have been highly coveted if he had wanted to go anywhere else given that he had 30 goals for England in just 38 appearances. As for trophies… he won the League War Cup & a solitary FA Cup.