It's actually rare for me to hear a song I outright dislike. Rare, but not impossible...
https://www.youtube.com/watch?v=LiaYDPRedWQ
I don't know if that's the worst song I've ever heard, necessarily, but it's the first thing that sprang to mind when I saw this thread.
I have to second this. Ironically, Avril Lavigne is one of my favorite singers because of her post-grunge and nu-metal days (her first two albums), and some of her other songs after that, like Head Above Water and When You're Gone, among a few others. But "Hello Kitty"... it blows my mind that the same person who wrote a song about the passing of a loved one, "Slipped Away", could write something like "Hello Kitty". I feel similarly about her song "Bad Girl" featuring Marilyn Manson... poorly written and cringy.
Other than that for me, literally any party song by Hollywood Undead, like "Coming in Hot", "No. 5", and "Everywhere I Go". I don't know why old Hollywood Undead enjoy these songs - the cringe factor aside, the lyrics are too crude for my taste, and I wouldn't recommend that anybody listen to them - the lyrics are really cringe and disturbing so I won't even link them here. Hollywood Undead's deeper songs like "Young", "We Are", "Pain", "Gotta Let Go", "Nightmare", and "Outside" have so much more lyrical depth and effort to them than their party-oriented songs ever will (the only exception to this in their 15 years as a band is their song "My Town").