Not just McClellan. There was a procession of horrible generals starting with Scott, McDowell (then McClellan) Burnside, Hooker, and Halleck. In my opinion Burnside was the worst with his Battle of Fredericksburg disaster. All those guys planned for weeks, then went into a big battle and usually lost because Lee had time to plan, move troops into place, and steer the battle toward his chosen topography. Grant was different. He believed in having a battle on Monday. Win, lose or draw, he went back into battle on Wednesday. He was a badass.