SAF era-
Stam - huge mistake, our defence was in disarray after he left, definitely cost us the league and probably the CL in 2001/02. we signed Veron and Ruud that season but eveything was destroyed due to our incompetency to defend. we signed Rio a year later which won us the league again in 2003 but Stam-Rio for a few years before Vidic arrived would have been great.
Tevez- it was a shame that our new holy trinity last for only a season (2007/08 season) as Berba's arrival kind of broke the partnership upfront that won us the league and CL. we would have defend the CL if not for the great Barca side anyway, but not signing Tevez and losing Ronaldo the same summer cost us five consecutive league tittles, as I am convinced we would have won the league in 2010 as well with Tevez, Rooney and Berba as our strikers.
Beckham- this is more about sentimental thing actually. my favourite United player. we would still signed Ronaldo even if Beckham stayed, i think.
post SAF-
Zaha- at least he is worth a try at RW, we haven't got a proper right winger for more than 6 years now.
Nani- to think that we was part of the Rojo transfer deal
. again he could play on the right if Zaha can't succeed at United.
Evans- he is better than Jones, Rojo, Bailly (because he is injury prone) and maybe even Lindelof.
Rio/Vidic/Evra - losing all those experience in the same summer.