Some of the worst antagonists in an otherwise readable novel would have to be the South African time travelers in Harry Turtledove’s Guns of the South.
As a plot device to give the Confederates AKs, they work. As fighting antagonists once they turn on their former patrons and teach them why small arms are still at the bottom of the modern war food chain, they also work. As characters, they are utter failures.
In terms of character development, they’re cartoonishly evil, so they can make the Confederates look better in race relations in comparison. In terms of competence, they choose the dumbest, clumsiest, and most backfiring way possible of trying to kill Robert E. Lee once relations sour.
They’re still not as bad as SCP-682, my personal least favorite villain of all time, but they’re definitely up there, especially since they’re not the products of an internet whim or a Bad Fiction Spotlight subject, but rather in an otherwise alright book.
(That this is one of Turtledove’s best books says something about him).