Well the worst gift I ever got from someone for my DAUGHTER, was a second hand dress that her daughter had grown out of, that stunk of cigarette smoke, and had a weird stain on it, (that I hope to God was beer.
) No label on it, or receipt (obviously.)
Needless to say, it went in the bin. Then I disinfected my hands!
Worst one I have ever received. A blank cassette tape from a cousin, for Christmas.
Over the years, various extended family members have given me (and my kids and DH) some real shit gifts........ From items that are obviously free gifts from magazines and newspapers, (like the Daily Mail free DVD's,) to second hand clothes they have obviously worn, to re-gifted boxes of chocs that still have the label addressed to them from someone else on it. 
@Horrordoervres
Not the worst but definitely the funniest - My great grandmother once gifted me a single bunny rabbit shaped slipper as a young teen, was super confused until I met my cousin at the family Christmas dinner and was informed she had received the other slipper.. - long story short my Great G got confused thinking they were teddies and she'd gotten a great bargain 2 for 1 deal in the store, got her eyes tested after that one.
That's fukin' brilliant!
Bless her little heart!