I don’t know who would be on that list. The three I thought might be frontrunners - Halleck, Fremont and McClellan - all resigned from the army before 1858 and didn’t return to the service until the war started. No wonder Lee was the first choice of the War Dept to be commander in chief.
Lee was the obvious choice for both sides, and his conduct during the war showed that he wasn’t overrated. As for the rest, there were a number of officers of mid-grade rank left to the Union. But so many “top” generals were southern, like Lee, Beauregard and Bragg, that the Union had to go second tier for many of their generals. So you wound up with a parade of guys like Halleck, McDowell and Banks. McClellan made his name early in West Virginia, otherwise he was not highly rated going into the war.
For the Union, it really was a matter of Lincoln going through a bunch of guys who couldn’t hack it until he finally wound up with winners like Grant, Sherman, Sheridan and Thomas.