There's no doubting Marshall's importance in winning the Second World War, but frankly, as he never actually commanded troops in combat as a general during any war, you can't really rate him against other combat commanders.
No implication on my part. A simple statement. Lee was likely the most talented tactician of the war, and one of the better generals of the war on the operational level. On the other hand, his grasp of the larger strategic aspects of the war were rather more limited. No, he wasn't the CSA Commander (that was Davis himself), but the Antietam and Gettysburg campaigns both showed that he didn't fully understand Washington's lesson of "An Army in Being", and he was as parochial as any other General in protecting his turf (the ANV as opposed to the rest of the splintering CSA.) He was good to be sure, but I absolutely put Grant better and likely Sherman among ACW generals. He was a better battlefield commander than either (especially Sherman) but the others both had better understandings of the overall realities of the war.
McClellan is a very interesting discussion. He had his limits to be sure, but frankly, he wasn't nearly as bad as saying he would be "black". He was beyond cautious to a fault absolutely (he may not have invented camping, but he certainly perfected it.) At the Seven Days he left too much to his subordinates, and you can even make a decent argument for Treason at 2nd Bull Run. Yet, for all that, the plan for the Peninsula was well formed, he moved fairly effectively pinning down Lee at Antietam. Sure, he got the break of the lost orders, but he still made somewhat good use of them.
The generals I put below him? They are the very definition of militantly bad to my mind. Actual drags on their armies.
Earl Van Dorn. Confederate Commander in Arkansas early in the ACW. Attempted to attack the Union at Pea Ridge, divided his command on two sides of a mountain and tried to attack on both sides. The side he wasn't with had no effective leadership and just sort of meandered. His side pushed but not enough to win the battle. However, to gain even that position, he'd entirely cut himself off from his ammo train (while driving Curtis back onto his own.) When day 2 of the battle began, the Confederates found themselves forced to retreat very quickly from lack of ammo.
Then, he abandoned his position in Arkansas, effectively abandoning the entire state to the Union to go fight in Mississippi. Beat his head into the wall against Rosecrans at Corinth. Eventually killed by a jealous husband. (Doesn't directly relate to his generalship, but humorous.)
James Wilkinson: Where does one begin? A traitor in the pay of Spain. Then, in the War of 1812, refused to cooperate with another nearby US force because he and the commander had a long standing feud. Spent the entire Chrysler's Farm campaign strung out on opiates, and ended up having to drag back to his base. As red as red can be.