Note: This only covers within my lifetime and maybe a couple years before, since I'm not hunting back to the WW2 era or before for this.
Minnesota Vikings: Les Steckel, the lone black mark on a franchise with otherwise respectable coaching history (hell, even Jerry Burns made it to the NFC championship). Before anyone else says Mike Tice, I'm willing to give him another chance given the roster the last couple of years. If he fucks up again though...
Cincinnati Reds: Pete Rose. Arguments about betting aside, he was a Hall of fame player, but a shitty manager.
Boston Red Sox: I'm a relatively recent Sox fan (last couple of years) so I'll let a more experienced Sox fan field this one, although I'd say John MacNamara if only for the stupid decision of putting a weak-kneed Bill Buckner out on the field.
Michigan football: Gary Moeller, if only for the supposed bar incident. He was a good coach otherwise in his short tenure (two Rose Bowl appearances and one victory)
Kansas State football: Anyone not named Bill Snyder