10. Owen Hargreaves | £22.50m
Transfer: Bayern Munich to Man United, June 2007
Despite suffering with injury problems at Bayern Munich, Ferguson decided to splash out £22.50m on Hargreaves in 2007.
Those issues continued at United, unfortunately. He made 39 appearances for United, with just five of those coming in his last three seasons at the club.
He was a decent player who sadly could never stay off the treatment table.
Rating: 2/10
9. Nani | £22.95m
Transfer: Sporting CP to Man United, July 2007
Nani showed flashes of brilliance during his eight-year spell at United. In total, he scored 40 goals in 230 games.
You can't help but feel he didn't quite fulfil his potential, though.
Rating: 6.5/10
8. Michael Carrick | £24.48m
Transfer: Tottenham to Man United, July 2006
Carrick was a classy player who didn't always get the respected he deserved.
In total, he featured in 464 games for the Red Devils, scoring 24 goals.
He helped United to 17 trophies, including five Premier League titles.
Rating: 8.5/10
7. Ruud van Nistelrooy | £25.65m
Transfer: PSV to Man United, June 2001
Van Nistelrooy was lethal in front of goal for United, scoring 150 times in 219 games.
The Dutchman had a publicised falling out in 2006 and he ended up joining Real Madrid a few months later.
Rating: 9.5/10
6. Robin van Persie | £27.63m
Transfer: Arsenal to Man United, August 2012
Van Persie almost single-handedly won the Premier League title in Alex Ferguson's last season in charge, scoring 26 times.
He couldn't replicate that form though and left the club in 2015 having scored 58 times in 105 games.
Rating: 8/10
5. Anderson | £28.35m
Transfer: Porto to Man United, July 2007
Signed as a teenager, Anderson won the Golden Ball just a year after joining United.
He had so much promise but he never managed to push on and failed to establish himself as a first-team regular.
The Brazilian left in 2015 having played 181 games and scored nine times.
Rating: 4/10
4. Wayne Rooney | £33.30m
Transfer: Everton to Man United, August 2004
It was clear Rooney was going to be a world star when he made his debut for Everton in 2002.
Ferguson won the race for his signature in 2004, paying a world record fee for a teenager.
Rooney went on to score 253 goals for the club across 13 years. He is undoubtedly one of the best players in United's history.
Rating: 10/10
3. Dimitar Berbatov | £34.20m
Transfer: Tottenham to Man United, August 2008
Berbatov lasted four seasons at the club, scoring 56 times.
He won the Premier League Golden Boot in the 2010/11 season and helped United to six major trophies.
Rating: 7.5/10
2. Juan Sebastian Veron | £38.34m
Transfer: Lazio to Man United, July 2001
Veron became English football's most expensive player when he signed for £38m in 2001.
He didn't live up to expectations, though. After a good start, Veron fell away and never showed why United paid so much money for him.
Just two years after joining United, Ferguson decided to cut his losses and sold him to Chelsea for £15 million.
Rating: 3/10
1. Rio Ferdinand | £41.40m
Transfer: Leeds to Man United, July 2002
Ferdinand came at a high price, with Ferguson smashing the world record to make him football's most expensive defender in 2002.
The Englishman paid that fee back and more. Ferdinand lasted 12 years at the club, making 455 appearances and scoring eight times.
He may well be the best defender in United's history.
Rating: 10/10
Average rating: 6.4
Ferguson spent a lot during his career. While he may be the greatest manager ever, he didn't always spend his money wisely.
Of his 25 most expensive signings, we've given ratings of three or below to five of them.
While signing Ronaldo and Rooney were masterstrokes, spending such great amounts of money on the likes of Veron and Anderson was disastrous.