#1 Claude Makelele

Claude Makelele's sale, in order to accommodate the iconic David Beckham, is the benchmark of Real Madrid's stupidity and a lacklustre transfer policy. Even Zinedine Zidane blamed the club for letting his teammate join Chelsea, a decision that further reaped losses for the giants.
Florentino Perez couldn't have been more naive in saying that Claude Makelele was an average player who failed to pass the ball three metres.
The French anchorman, in turn, made sure Perez ate his words after dazzling for Chelsea under Jose Mourinho. He is attributed to redefining the role of a modern defensive midfielder, named as 'The Makelele Role' after him.
Makelele's departure saw Real crumble whereas the Frenchman became the architect of Jose Mourinho's title-winning squad. Los Blancos' policy of sacrificing defensive players and replacing them with attackers later paid the price, as the power shifted to Barcelona in Spain.
Makelele's sale has been touted as the biggest transfer error of Real Madrid by many in the football world. The club has improved since then, signing the likes of Xabi Alonso and Sami Khedira. But, do they sit at the same table as Claude Makelele? Debatable.