Sir Alex Ferguson
Sir Alex Ferguson is not only a United legend. But a legend of football, and a sporting legend, period. He is possibly the best football manager of all-time. His tenure from 1986-2013 brought immense success to the club, and helped United further its global standing, and boost the Premier League's appeal.
This isn't to be disrespectful to Sir Alex's legacy or tenure. But his successor was David Moyes, who whilst performing well at Everton (a big and traditional club in its own right), he was unprepared to manage United, the biggest club in the United Kingdom.
Moreover, United at that point were champions. And it was always going to be hard for him. There is a strong case that Sir Alex himself chose Moyes to succeed him. This may have been well-intentioned. Though he apparently appeared out of his depth.
This may seem harsh to cite, overall. However, possibly a manager with more experience could have been better.