At one time I would had said Russia, hands down. But the food in Mainland China, the sort eaten by the mass of people every day, may in fact and contrary to expectations be the “worst” in the world. Besides the country’s infamous food quality non-enforcement (shredded cardboard in meat pies for example) the food in the PRC is very far from what’s available in all the surrounding Chinese speaking countries like Hong Kong and Taiwan. Food served in high class restaurants in the big cities can still be outstanding but is out of reach for most people. The meat ingredients are 90 percent bone and gristle and only in there to add flavor. Rice and noodles make up the bulk of calories people consume.
You're describing the McDonalds down at the corner. I refuse to eat there, but friend "treated me" to their drive through a couple of weeks ago. Had an alleged Chicken WRap. No way was there any chicken in those imitation meat hunks. Unless perhaps ground up and extruded beaks, wattles, eyeballs and cloacas. Disgusting.
Therefore, I'm voting for the USA as having the worst food. Close the fast food joints, is all I can recommend.