In the second of our fourt part series to celebrate Sir Alex Ferguson's 21 years at Old Trafford, James Robson lists the Old Trafford boss's best signings.

21 Brian McClair, 1987-98, £850,000 - Not everyone's cup of tea, but whether as a striker, midfielder or coach, he was a consummate servant to the club.

20 Henrik Larsson, Jan-Feb 2007, loan - A surprise inclusion, but Larsson's role in United's title success last season should not be underestimated. Arriving at a time when they were down to the bare bones in attack, he was on the losing side only once in 13 games. Had he stayed for more longer, United may well have stayed the course in the Champions League as well.

19 Nemanja Vidic, 2006-present, £7m - The Serbian is in the early days of his United career, but has already made a big impression. Strong as an ox, he brings back memories of Steve Bruce in both his bravery and eye for goal.

18 Edwin Van der Sar 2005-present, undisclosed - Fergie has long struggled to find a replacement for Peter Schmeichel and Van der Sar is as close as he's come.

17 Dwight Yorke, 1998-2002, £12.6m - The most frustrating thing about Yorke is never knowing how good he could have been. At least United fans will always have the memories of his outstanding debut treble season.

16 Rio Ferdinand, 2002-present, £30m - Criticised for his laid-back approach, but Ferdinand is a classy defender and creator from the back. Helped United to the title in his first season at the club - and his suspension midway through the next campaign pulled the rug from under their defence of it.

15 Teddy Sheringham, 1997-2001, £3.5m - Aged 31, Eric Cantona announced his retirement in '97. Teddy Sheringham was the same age when signed as his replacement later that summer. Looking for medals to crown a hitherto commendable career, he left United four years later with three titles, an FA Cup and the Champions League. Job done.

14 Andy Cole, 1995-2001, £7m - The ex-Newcastle hitman was a constant focus of criticism, not least from former England manager Glenn Hoddle. But in six years at Old Trafford he won five titles, two FA Cups and the Champions League - scoring 121 goals in 275 appearances.

13 Jaap Stam, 1998-2001, £10.75m - Described as a colossus on skates, the Dutchman had power and pace - and was the bedrock of United's treble success in '99. Won the title in each of his three seasons at Old Trafford. Had he not been hastily sold in 2001, United might have gone on to be crowned champions for a fourth successive year.

12 Wayne Rooney, 2004-present, £27m - After missing out on Paul Gascoigne, the mercurial English talent of his generation, there was no way Ferguson was going to let another, Rooney, slip through his grasp. Fresh from an outstanding Euro 2004, the then Everton striker was the most sought-after young player in Europe when United splashed out £27m for his services. He responded by scoring a hat trick on his debut and becoming a pivotal figure for both club and country. Should be much higher up the list when he eventually calls time on his career.

11 Gary Pallister, 1989-1998, £2.3m - Ferguson was prepared to break the British transfer record for a defender when signing Pallister for £2.3m from Middlesbrough. Had a nightmare debut - 2-0 defeat to Norwich - but rarely put a foot wrong thereafter.

10 Cristiano Ronaldo, 2003-present, £12m - Signed as a replacement for David Beckham in 2003, Ronaldo announced himself with a dazzling cameo on his debut against Bolton. He has since matured into one of the best players in the world and fired United to their first title in four years last season.

9 Steve Bruce, 1987-1996, £800,000 - Ferguson was stunned that United had allowed Terry Butcher to leave Ipswich for Rangers when he took up the post in '86. Seeking a similarly selfless warrior for the heart of his defence, he settled on Norwich's Steve Bruce and never looked back.

8 Mark Hughes, 1988-1995, £1.8m - The Welshman was sold to Barcelona the season before Ferguson took the job at United, but he wasted little time in bringing him back to the club in 88. Hughes consistently scored outrageous and vital goals. His double against Crystal Palace in the '90 FA Cup final springs to mind, as do his strikes against Barcelona in the Cup Winners' Cup final and a last-gasp equaliser against Oldham in the '94 FA Cup semi.

7 Ole Gunnar Solskjaer, 1996-2007, £1.5m - At £1.5m the baby-faced assassin was an absolute steal. So much more than a supersub, but he had an uncanny knack of coming off the bench to score vital goals. Not least the winner in the Champions League final.

6 Ruud van Nistelrooy, 2001-2006, £19m - Ferguson was prepared to wait for the deadly Dutchman to overcome knee-ligament damage - and how he was rewarded. Van Nistelrooy was the goal machine he had longed for throughout his time at Old Trafford. With 150 goals in 217 appearances, his record compares with United greats Bobby Charlton and Denis Law.

5 Denis Irwin, 1990-2002, £625,000 - The unassuming Irishman was virtually faultless at left-back for the best part of 10 years. Not bad considering he was right-footed. Perhaps the most consistent performer throughout Fergie's reign. He also possessed a mean free-kick with either foot.

4 Ryan Giggs, 1990-present, trainee - Technically a product the United youth system that also gave him David Beckham, Paul Scholes, Nicky Butt and the Neville brothers, but Ferguson personally paid Giggs a house call to ensure he tied his future to the club when still a schoolboy. The Welshman has been the one constant throughout United's nine titles since then. Giggs has been peerless on the left for more than 15 years. Should go on to break Bobby Charlton's record of 759 Reds appearances.

3 Roy Keane, 1993-2005, £3.75m - The Irishman had big boots to fill when signed as a successor to Bryan Robson in '93, but he took that mantle in his stride. Incomparable in central midfield throughout the 90s, he was the heartbeat behind Ferguson's glory years.

2 Peter Schmeichel, 1991-1999, £550,000 - Schmeichel is not only United's greatest-ever goalkeeper, he might just be the finest keeper in the history of the game. The Great Dane cost only £550,000 when signed from Brondby in 91, which Fergie described as the "bargain of the century". Won every club honour going at United - captaining them to Champions League glory in his final appearance for the club.

1 Eric Cantona, 1992-1997, £1.2m - An obvious choice, but without him, who knows how long United would have been searching an elusive league title? Cantona was the crowning glory of a team Ferguson had been building since taking charge in 86 - and the catalyst to the success that followed. In four and a half seasons at United he won four titles and two doubles and inspired the golden generation of Giggs, Beckham and Scholes. Stature, majesty, charisma, he became a legend after joining from Leeds for £1.2m. Cheap at 10 times the price.

Fergie has made a few mistakes too...Massimo Taibi, 1999-2000 £4.5m - Fergie's success with goalkeepers has been mixed to say the least, but none flopped so badly as Taibi. Eric Djemba-Djemba, 2003-2005, £3.5m - The Cameroon international was seen as a potential successor to Roy Keane when he arrived. But not for long. Kleberson, 2003-2005, £5.93m - United spent the summer of 2003 chasing Ronaldinho, but ended up with his Brazilian compatriot Kleberson. Nationality was the only similarity they shared.

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