Marshall to me was ahead of the rest, but only marginally so.
Once you take into account that he had every conceivable advantage that could be afforded to an ATG pacer (shorter career concentrated on peak, worldclass pace support, poorer batting opposition, great scoreboard pressure) then the gloss around his stats begins to lessen somewhat.
In all likelihood, he would have been a 21-23 averaging pacer with a 50 or so SR. Of course, impossible to tell for sure but we make these mental adjustments with records of batsmen all the time.
Also worth mentioning that for those really trying to make him part of a trinity with Bradman and Sobers, remember that in his career he wasnt really hyped or feted ever to that degree of a Lillee, Viv, Warne, Lara or Tendulkar. Can be partially explained by the fact that there might have been pace exhaustion by the time he peaked in world cricket but it is true.
@kyear2 goes on about certain ATG XI, but you will struggle to find lists where he is put in the top ten.
Of course, he was generally rated the best pacer of his time, no doubt, but even when you listen to those comments from his peers, it is never that he was greatly better than all other pacers but his skills on different wickets gave him an edge.
One can make case for rating McGrath best on longevity or others but I agree Marshall has the best case but he is not a tier ahead of anybody.
Lastly, was watching this clip of late career Marshall bowling. He was canny and skilled though he had lost his pace and it was funny to see Benaud start the clip by calling the best bowler of all-time a 'good bowler'