Since so many people on this site seem to only eat "fast foods," I'm going to assume you mean WORST fast food restaurant. For example, I once ate at a place called "Court of the Shadows" in Miami that (accidently) featured the head of a dead rat in a frittata. At least I hope it was an accident. But that information is of very limited use unless you live in Miami.
So, to focus on national food chains, my answer would be the Cheesecake Factory. While the actual food isn't all that bad, they are the ones that apparently started the trend towards ENORMOUS individual servings that have worked to make Americans the fattest people on earth. And the Cracker Barrel, with their bigoted management (gay folks aren't welcome to work there or dine there) is close to being the very worst, no matter how many times old people vote it "best" in Consumer Reports magazine.
But again, neither of those are "fast food" restaurants. Hmm. Perhaps the worst of the worst of those is Jack in the Box. Funny ads, awful food, awful service.