#4 Ruud van Nistelrooy
The Dutch striker was really in his element at United and produced numerous great displays over the five years that he was there, plundering a hatful of goals - 150 in 219 appearances to be exact. That is a phenomenal scoring rate, especially after he suffered what would normally be classed as a career-threatening injury.
However, after falling out with Sir Alex Ferguson due to poor form and his criticisms over fledgeling superstar Cristiano Ronaldo, he left in 2006 for Real Madrid. Sadly, he never really recaptured the heights that he hit at United with his other clubs.
At Madrid, he was in and out of the team and his scoring rate gradually declined and he missed out on being in the Netherlands squad on a regular basis. Eventually, he wound his career down with comparative minnows Hamburg and Malaga.
Van Nistelrooy himself admitted his behaviour had been unacceptable in his latter years at United, even ringing Ferguson to apologise - a tale he recounted in his autobiography. And frankly, Van Nistelrooy is right to have regrets,
If he had stayed at United, he could have become the all-time top goal scorer, the mantle of which was instead taken by Wayne Rooney.