“Best“ for me is the sum of attributes like performance, age, injury proneness, tactical importance, even market value and image importance. On this merit, I agree with OP on Rashford.
3rd - I’m torn: Martial (best striker last season), Maguire (tactically most important after Bruno), Fred (win rate, midfield combat), AWB - best defensive full back in the world with tons of potential. So, I will treat them as tiers.
4rth - again, fierce competition Shaw, MCT, Greenwood, Lindelof.
It is a sign of a very classy team.