The dead cat from the "Jester's Bag" is probably the most useless as you cannot even sell it for a gold piece, but a couple of close seconds IMHO are the: "Ring of Pain Amplification"
you get off of the "High Torturer" in Dragon's Eye. or the "Gauntlets of Infernal Damnation"
found in a treasure trove in the Frost Giant Cavern..
