I agree with a lot of what's already been said...
Tony's in the MK is inedible. Blech. Over at DHS, I also avoid Mama Melrose. Whoever the chef is there has an extreme fondness for the salt-shaker. At Epcot, I won't ever go back to the Coral Reef- had a few bad meals there, in addition to horrendously slow service. As far as resort dininng, given the price they charge for it, I found the dinner at Chef Mickey's to be a complete waste. Terrible food (that looked like it had been sitting out there for hours) coupled with a disorganized environment.
For counter service, the worst has to be the ABC Commissary, though dishonorable mention also goes to Electric Umbrella, Village Haus, and Pizza Planet.