I think people are forgetting how pleasing Martial's first season was, given many of us knew little about him when he first joined and he immediately picked up the slack of RVP and Chicharito declining and departing, even outscoring Rooney. The problem is that was his best season until this one. He was 19 at the start of that season.
Rashford came in at a more difficult time later in LVG's second season. He got that pair of braces early which got everyone very excited, but was then playing as almost a glorified wing back in some games for a very passive LVG team. Rashford looked a prospect, but his knack of scoring on debut masked him still being very raw too. He's improved massively since then. He was 17 at the start of that season, 18 by debut.
Until recently, Greenwood has been another young player whose knack of scoring masks his otherwise fairly underwhelming underlying stats. The talent is clearly there, especially when he gets a shot at goal, which is so regularly gets off given half a chance, but he’s disappeared at centre forward often, and looked a square peg in a round hole out wide. The last two games have been the best of his career, looking a genuine handful, and he’s benefited from being in an in-form attacking team. He was 17 at the start of this season.
I think Rashford will likely have the best career. He has shown the most growth so far, which stands him in good stead for periods out of form. Martial is somewhat at what I expect will prove to be his peak. Mason has made a great start, but is still relatively unproven and has faced virtually zero pressure so far.