Taco Bell has to be the Numero Uno!...................
One time at Taco Smell I got my food on a tray. It turns out that they didn't even bother to wipe the remains of the previous user's meal off the tray, much less wash it.
That was bad enough, but my worst ever experience was at a Burglar King. I went into the rest room to wash my hands. There were no paper towels so I went into the stall to get some toilet paper. There was none, then I observed that someone had smeared excrement all over the walls of the stall. I complained to the manager and walked out.