A thread like this has the potential to unfairly label a popular restaurant providing happy meals to thousands (literally) of patrons on an annual basis.
I've enjoyed meals at practically every place on this list.
If there is a place (or 5) where I'd tend to shy away from, it would be because of my personal tastes and the food allergies of people in my group. At the end of the day, if there was any restaurant that consistently provided a sub-par experience to its patrons, it wouldn't last.
That said, my least favorites are:
Liberty Tree Tavern, Magic Kingdom. I find it to be way overpriced and the food at dinner is too heavy for me when I am going to leave there and make my way around the parks. I think it used to be a character meal spot and they never lowered the prices after Mickey and friends vacated but they probably should have.
Marrakesh, Epcot. I found this restaurant sent really mixed messages. They sold American beer and - correct me if I am wrong - you can't buy alcoholic beverages at restaurants in Morocco? We were seated during the middle of the live performance which never re-started before we finished our meal, so that was kind of disappointing. There was another factor, uniqure to our dining experience, that I won't mention here but suffice to say it could have happened anywhere but the fact it happened at this place when we visited for the first and only time left a bad memory.
Planet Hollywood, DTD. Again, price versus value. There was nothing wrong with the food but the memorabilia didn't justify their price for a hamburger for me, personally. The server made a fatal error that has resulted in our not returning in over a decade. But it is still there, so they are obviously doing something right.
Hollywood and Vine, Studios. The first time we ate at this buffet, we were surprised at how great it was. The second time, it missed the mark badly. I guess that is the risk of a buffet. Uneven food quality? There was another problem on that second visit that resulted in a manager's intervention to clear things up, which was appreciated. Next time, we will try a different place.
and fifth - there really isn't a fifth. I looked over the list others have posted here and can't really say that my experience with the other places has been unsatisfactory.