The worst thing I ever ate was room service pizza from the Contemporary. Just too doughy and saucy and pretty much tasteless. I would say most pizza from Disney could be described this way. Then again, I'm from Chicago and have access to really great pizza at home. So I just don't order it anymore. We were just exhausted one night after getting back to BLT from the parks and 24 hour room service was pretty much our only option and pizza wasn't going to break the bank. Ah, but you get what you pay for.
2nd place goes to a cream cheese pretzel. The pretzel was hard as a rock and the cream cheese just oozed out. And it was sickeningly sweet.
Finally, if you like the turkey legs at Disney, do not continue reading (same goes if you are easily grossed out.) I've never had a turkey leg at Disney, but many, many years ago a co-worker ordered one at the Taste of Chicago. It hadn't been cooked enough, and when she bit into it, a tendon burst and blood just spurted out. It was beyond scary. I don't care how good those turkey legs might be, I'll just never get past that sight. No turkey legs for me - ever.