The worst gift is a gift that had no thought at all put in, and just regarded as an 'entry requirement' and a social obligation rather than a labour of love, friendship and thoughtfulness.
When I was a kid, one of my classmates' came to my party with a tiny present, badly wrapped in some cheap wrapping that looking like a torn away wallpaper.
He made a big ruckus in the party, making stupid jokes, cracking dumb antics, interrupting all the games, ate the most in the party in the most crude manner with no proper ettiquette or proper teaching, demanded for seconds, thirds and fourths and even took the other kids' food from their plate.
I yelled at him to get the hell out of my party after the meal and he left indignantly: I was never affected by social sigma or whatever other kids thought of me. I was whatever I wanted to be, so needless to say the big bully realised his size didn't faze me, a little girl.
Later, when I opened the present, it was an old Mcdonalds toy given away in the happy meal, already opened, played with and obviously a discard.
A very horrible gift, from a sorry little monger.