OH BOY. That's quite a question. The absolute worst fanfics tend to be:
Truth or dare fanfictions (where an author stand-in with godlike powers forces characters to play truth or dare),
Shipping fanfictions, and especially slash fanfictions tend to be really OOC,
and the "un-parody" fanfics, as I like to call them, when an author writes a bunch of random crap and then claims it's a "parody." Many fanfictions with plots like, "X and Y turn into goldfish," or "X goes to the grocery store" are this.