Of the ACW bunch...
What about William Loring? CSA, Division commander, Army of Mississippi. Pretty much single-handedly prevented Pemberton from making any kind of effective defense of Vicksburg by early in the campaign repeatedly disobeying orders and refusing to attack where he locally outnumbered the enemy 2 to 1, then marching his division off of the field without informing Pemberton - in multiple battles! Which of course caused Pemberton to delay retreating to wait for Loring to catch up - which he wasn't - and so ensured he had no escape route.
Or James Ledlie? USA, Division commander, IX corps. Habitually drunk and during the Overland Campaign had his men charge repeatedly against strong fieldworks, more so than Grant ordered. But primarily, his was the lead division in the Battle of the Crater, which should be enough of a justification right there. And he was drunk. Again.
Of some of the other people mentioned: John Pope shone at New Madrid. Burnside performed competently at Knoxville and he knew his own limitations. Hooker was a good division commander plus his success at Lookout Mountain. Sigel - eh, others are worse. Banks similarly. Bragg could come up with good ideas, and at least won 1 battle in his career. Hood, we've had the argument before - he was a superb division commander, thus however poorly he performed with the AoT he cannot be the "worst". Leonidas Polk......actually, you'll get no arguments from me - he very well could be the worst.