i've always wondered this, there's been so many over the last few years, my top 5 would be, 1.Eric Cantona - To sign him from Leeds for £1 million pounds, at the time it was alot of money, he was given the number 7 shirt, he helped us on becoming one of the most successful clubs in the premier league and is consdiered a legend at Old trafford and rightfully so, in today's market he would be worth at least £25 million pounds at the very least possibly more. 2. Chicharito - We got him for a bargain at the reported £6 million pounds consdiering other clubs spent huge money on strikers, he's developed into a goalpoacher, he reminds me of a young Michael Owen, and i'm sure he learnt alot from him as well. I'm glad we got him, if he was playing for one of our rivals i would want us to sign him, he has to be consdiered as one of our best signings. 3. Peter Schemeichel - One of the best goalkeepers we've ever had at the club, he cost a couple of grand, he was a winner and also a United fan when he joined us which helps, i don't think we'll see another keeper like him again, he was crucial in helping us win our first league title in 26 years, he was instrumental, and a charcter as well, although he did play for Man City ( in my opinion he was an agent there for us ), i will always consider him a United legend. 4. Denis Irwin -he cost us £620,000 which in today's market is nothing infact its a bargain, he could play right-back or left-back and he scored alot of free kicks for us as well and the only player in the 90's that was part of both of our european success, he was underrated and should've got more credit for his performances in my opinion. 5. Edwin Van Der Sar - We should've got him when the great dane retired back in 1999 but we got him in 2005 for £2 million pounds and what a signing he was, he helped us to win the champions league since the treble winning season, saving Nicolas ANelka's penalty in the process, he went on a run of clean sheets in the 08/09 season and that helped us tie level with Liverpool on league titles, we could've won another 2 champions league's of we had got him all them years ago but that's football.
Matth, depends on which period we are talking here. My top 5 (off the top of my head) in no specific order as they say Cantona, Ronaldo, Robson, Schemeichel, and Best (if you can count him as a signing!) And I have not included the likes of Giggs Beckham and Scholes who were part of the youth academy.
Agreed, apparently he rejected Liverpool to join us, he's a world class defender, we missed him big time last season.