Sadly, the reasoning is behind the paywall for me.
Giggs, as great of a servant to the club he was (I'm purposefully ignoring all the stuff that came up in the later years even though it really damaged his legendary status in my eyes to say the least), is too high. Charlton's longevity is comparable while their individual contribution, well, isn't — Sir Bobby's path, from breaking in to the Busby Babes team, surviving the crash, getting back the same season to assume a position of leadership (aged only 20) and playing a key role in club's resurgence to the very top while also being one of the best players in the world at the time... He's always going to be the number 1 for me.
Obviously George Best embodies the most exciting part of United's image — flair and flamboyance at the brink of arrogance, same as Eric Cantona in the 90's (but, unlike Cantona, he's not just one of the best players in United's history, you can certainly make the case of George Best being one of the most talented footballers to ever grace the football pitch), so he has to be at the very top, #3 seems like a good spot for him.
Duncan Edwards is probably a little overrated and I'm not talking about his ability (I've been going through some 1950's footage lately and he always stands out when he's on the pitch), but he can't be higher than Roy Keane when we take a look at what they actually gave to this club. Yes, Edwards' name will always be linked to Busby Babes & Munich and he's the greatest "what if" not only in United's, but also in England's history, but he has to be a bit lower.
Underrated:
van der Sar (at least 10 places too low)
Gregg (more than just a player for United)
McGrath (mid 20's at the least)
Carrick (~30-ish)
Cole (I'd place him a good 5-7 places higher)
Stam (his stay here was way too short but I'd certainly say that he's a "greater" United player than, say, Pallister)
Overrated:
Stiles (such a lovely guy & a proper character but he shouldn't be ahead of the likes of Vidić, De Gea, van der Sar etc.)
Schmeichel (a little bit; for me, he's our greatest ever keeper but it's really close)
van Nistelrooy (maybe a little bit — too little team success even though individually he had been simply outstanding)