1: Cristiano Ronaldo
Those would would question Ronaldo's place at the summit of this list need to remind themselves just how good he had become by the time he left for Real Madrid.
The skinny teenager who signed for Sir Alex in 2003 developed into an incredible player, and was one of the best players of the world by 2009, winning three Premier Leagues, one Champions League and an FA Cup in the process.
At £12.24million he cost a little more than the others on this list, but considering United were buying a man who could turn out to be one of the best players to have ever kicked a football, that shouldn't be dwelled upon too much.
Ronaldo is a rare specimen, and Ferguson did extremely well to bring him to Old Trafford ahead of his rivals.