Real Madrid vs. Barcelona. Los Merengues vs. the Blaugrana. The Blonde Arrow vs. the Total Footballer.
Two of the greatest players in the history of the game, these two share the top spot because they both had such dramatic, game-changing impacts on the teams they loved, on the rivalry that defined them, on Spanish football and world football at large.
Alfredo Di Stéfano was the more significant transfer because of all of the conflict surrounding his signing with Real Madrid, but it seemed fitting to place these two together at the top of the list as bookends to one of football's greatest rivalries and as the essential representatives of two of its greatest dynasties.
The Deal: In 1953, Alfredo Di Stéfano was supposed to be heading to Barcelona from Colombian club Millonarios, but Millonarios never approved the transfer.
The deal was made with Barcelona and FIFA without the permission of the Colombian side, but the Spanish football governing body, needing their approval, blocked the transfer. Real Madrid capitalized on the tension in the situation and convinced Di Stéfano to sign with them.
After a contentious battle, FIFA declared that Di Stéfano would play four seasons in Spain, alternating between Real Madrid and Barcelona. The Blaugrana, dissatisfied with the decision, dropped their bid and Di Stéfano joined Los Merengues.
The deal also led to a temporary ban on foreign players transferring to La Liga.
The Impact: The battle over Di Stéfano galvanized Real and Barça supporters' ire towards one another and contributed to the passion that surrounds El Clásico to this very day.
But Di Stéfano's contributions to Real Madrid run much, much deeper.
He forged a partnership with striker Ferenc Puskás, and together the two became one of the most powerful offensive duos in football history, and his record for most goals scored in European competition stood until fellow Madridista Raúl broke it in 2005.
He scored more than 300 goals for the club, won eight La Liga titles and five consecutive European Cups. For his contributions, the Spanish sports newspaper Marca awards the best La Liga player of the year with the Alfredo Di Stéfano trophy.
The Deal: In 1973, Ajax star Johan Cruyff, who had already won three consecutive European Cups and eight Eredivisie titles with the Amsterdammers, moved to Barcelona for a fee of 6 million Dutch guilders ($2 million or £922,200).
The Impact: The total footballer became the total Culé. He didn't just absorb himself in the club, but he absorbed himself in all it stood for. He even gave his son a Catalan name, Jordi. And on the pitch, he gave Barcelona a 5-0 victory in El Clásico and led them to their first La Liga title in more than a decade and a Copa del Rey title.
He brought his mastery of 'Total Football' to the Catalan side, where evidence of it can still be seen in how the squad plays today, with its mastery of Cruyff's 'tiki-taka' passing technique.
AAnd as the ties to the club were created in his time on the pitch, he returned to make an even greater mark as a manager, netting the Blaugrana their first European Cup, four La Liga titles and a Copa del Rey.
He would be Barcelona's most successful manager until Josep Guardiola, who he signed as a player.