Martial has had a good season, but pales in comparison to the rest on the list.
He doesn't pale in comparison at all, Coutinho is a not great player. He scores a cracking goal then has 3-4 woeful games. He has taken 100 shots to get 8 goals. He's taken about 100 shots in each full season for Liverpool, he's wasteful as hell with one of the worse strike rates in the league of any player taking a fairly high number of shots(say over 30).
Barkley is equally inconsistent and also scored 8 but in 89 shots, 3rd full year in the league, wasteful, constantly makes the wrong decisions. They both usually play relatively central behind the striker and get to follow into the edge of the box onto loose balls and pick up goals, in fact I'd say the majority of Coutinho's goals in the past 4 years have been of that type.
Martial plays wide left, it's his first year in the league, only second full season, he's 2 years younger than Barkley and 2.5 younger than Coutinho. He also has 8 goal but in 52 shots. So that is a goals/shots ratio of 1:12.5 for Coutinho, 1:11.1 for Barkley, and 1:6.5 for Martial.
Martial has also been largely excellent in each and every game I've seen where as Barkley and Coutinho have had incredibly poor games multiple times this(and every) season. Both of the other two play for teams that have scored considerably more than Utd this year, neither of which have had to put up with Rooney. Martial in the same positions as the other two and with actual strikers and a fluid offence to play around would have destroyed them both in terms of total goals.
In his third full season in this league he will make Coutinho/Barkley as they are today look extremely poor in comparison.
Lukaku has been a beast almost the whole season but is again significantly more experienced and playing the role he loves in a team that loves attacking. Butland is hard to compare, he's been good but he's played 3 full seasons previously at other clubs and is 23, probably being over rated a bit as a 'young' player.
Kane and Alli have simply been exceptional in their positions. 10 goals from 74 shots and 9 assists in his first year in this league starting at 19 is phenomenal for Alli. Kane while excellent goal scoring and his overall movement, work rate are also excellent is still older with more experience at this level than Alli so the right person won.
But Martial should have been 3rd on the list, with all the things people consider negatives, inexperience, second full season, first year in this league, first year in a new country, he's been fantastic despite those things.
Coutinho and Barkley, in their third/fourth seasons in this league, older, way more experienced, they've been wasteful, selfish, make the wrong decisions constantly have woeful games far too often, neither should be on the list at all.
I'd probably put Origi or Can in ahead of Coutinho just looking at other players at Liverpool.