I'm not going to be any use for finding the lowest published baddie, but I know that for walking-around AC, a PC in armor can get to -15 with just the DMG or -16 with UA. Plate Mail +5 for base of -2, add a shield +5 to get to -8, 18 Dex* to -12, dusky rose ioun stone gets us to -13**, Cloak of...