Not exactly the worst gift, but certainly brought the most disappointing. Which I guess would technically count.
I remember when I was little I wanted a Chia Pet. They were all the rage at the time and I thought they were neat. So I asked my Grandma for one. It was all I wanted. I should point out this Grandma likes to show off her money to people...so I ended up with all these pricey things wrapped in paper and ribbons that occasionally cost more than the gift it covered like I did every Christmas with her. I got to one present, opened it, and there was the Chia Pet picture on the side of the box. And I screamed in excitement, "A Chia Pet! Thank you! Thank you!"
...Grandma goes "Um...look inside the box". I look inside the box and it was a cassette player and tapes. "Oh. Tapes. Thanks, Grandma". I look back on that and think "That was a really crappy thing to do!" She knew I wanted the Chia Pet and she puts this expensive gift inside an empty Chia Pet box. What was she thinking?