Sumner was not a "crony" of McClellan's, certainly not
One of his cronies, no doubt. Sumner or Porter, perhaps? I am not sure about the relevant seniority.
Sumner was not a "crony" of McClellan's, certainly not in the sense of FJ Porter, Franklin, or Burnside, all of whom got corps commands at GBM's request.
Sumner was a fighter and - in a real sense - was senior, although McDowell was in terms of date of rank. After them, in terms of officers with the Army of the Potomac, it would have been Heintzelman and then Keyes.
A dark horse is JKF Mansfield, who as Scott's inspector general pre-war, certainly had deeper knowledge of the army and officer corps as institutions than anyone else, had been sucessful as the initial commander of the Military District of Washington and been sucessful, had a relationship with Lincoln, and certainly was ready to fight.
Other (really) dark horses but with the seniority would include Wool and Dix, although Wool was a fighter, he was also
very old; Dix and (for example) Hitchcock would both have made capable chiefs of staff, and although Dix's command in Maryland was important, he was not the only man who could have filled that assignment.
Banks and Butler had rank, but no one would have entrusted an army to either of them at the time. Both actually could have made fairly solid staff officers, however, despite their generally checkered reputations; both were capable enough administrators, and Banks as provost marshal general and Butler as judge advocate general certainly seem better uses of their respective skills. Fremont had already fumbled the West, Halleck was needed there, and the other '61 major generals (Cadwalader and Morgan, IIRC) were needed in their respective spheres.
Buell, Grant, Rosecrans, Pope, Curtis, and TW Sherman were all pretty much needed as theater or detached commanders at this point.
Best,