The WORST one I can think of was in DCA from the Lucky Fortune. Maybe I just got a bad one, but the Teriyaki Chicken bowl I got was like 99% rice. Most everything else at DL/DCA I've tried is at least okay if not pretty good, although I don't consider myself a huge foodie. The BEST one lately is the gray stuff (or now Summer Stuff). I don't eat that many sweets, but that is one that I make an exception for and indulge every visit.
Now if you want WORST EVER theme park food that one is easy. Hamburger at SeaWorld San Diego. You wouldn't think you could screw up a $20+ burger so badly, but that thing was absolutely inedible. The "meat" patty did not even look like something for human consumption. Took one small bite, spit that out, and threw the whole thing away and was put off food for days.