It's tough to pick just one, as, ideally, we need to strengthen in all those areas. But for me, the order is DM, Striker, CB, RW.
We definitely need a quality DM and, if we got one, it would help out our already decent CB options without having to add a new CB (which, ideally, we would as well).
In attack, we've got options for the wide areas - Rashford, Greenwood, Martial, James, Amad, Pellistri, Shoretire, etc. And that would be a decent bunch fighting for those positions, as well as some of them providing cover in the #9 role, so long as as can add a top quality central striker as first choice. Again, ideally, given the youthful nature of most of those listed wide players, we'd add one more experienced RW as first choice, but it's not as big a priority as a goalscoring #9.
So, again, for me the first priorities are DM and central striker.