I would go so far as to say there are none - there are players who are dead cert in terms of ability - but injuries, mentality or other things etc makes it impossible to say for certain.
During my time many of the biggest talents have all gone to waste due to off-the pitch incidents. Rafael Burke who was a bigger talent than Giggs lacked the ambition and drive, Greenwood got arrested, Morrison got involved with a lot of other stuff.
I would say Mainoo and Garnacho are the only 2 I am pretty certain will make it -if they have a normal progression - but...IF - they are head and shoulders above everyone else in terms of ability. Look at Elanga - he was by far our best reserve-player if we go back 18 months - he will go far, but to be a regular at United ? Probably not.