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?
Thomas Muller (an honourable mention not the greatest), plyed in Bayern Munich and has never has shown unwavering loyalty. I understand Bayern is a top club however even when he’s gotten fewer minutes, he’s still remained loyal. Even when he’s not on the pitch, Radiomuller is the spirit of the club, he represents us!