Ronaldo & van Basten are the obvious top-2 for me (and for almost everyone, I'd imagine). From the United greats Robson is the obvious choice and McGrath is another one — I think he would've cemented his place alongside Rio in United's All-Time XI if not for the injuries (and drinking).
The established XI (for some it's constant injuries, for others it's one career-defining one, but it's still a huge "what if?" question on top of an already amazing career):
Ronaldo - van Basten
Del Piero ------------------------ Robben
Robson - Schuster
Camacho* - McGrath - Sammer - Hargreaves*
Čech
Bench: Baggio, Fowler, Falcao, Torres, Owen, Thiago, Reus, Ballack, Redondo, Fabregas, Kaka, Cazorla...
*I really struggled with fullbacks, any ideas? It has to be an established superstar.
The early promise XI (very United-centric):
Lentini - Pato - Fati
Wilshere - Diaby
Rodwell
Fabio - Jones - Bailly - Rafael
Adler
Suggestions are welcome!