I can't remember the worst, but the Good Times chain in Colorado is my favourite fast food place of all time and the burger I always get is the Big Daddy Bacon Cheeseburger:
Sooo delicious but sooo bad for me! At least it's local produce o_O. They have the best fries and amazing milkshakes too, so good!
But I think one of my worse moments was when I was 17 and I dipped a Mars bar into a mug of hot chocolate. That was pretty disgusting.