The worst has to be the removal of the original Journey into Imagination, followed by Horizons, the removal of the old World of Motion, and the Tiki Room under New management. Oh, and just to show my age, I still miss If you had wings.
Two of the worst are gone now. . . 20000 Leagues under the Sea and the old Skyway. I think the worst of the current ones are Peter Pan or Dumbo. The Speedway can be bad as well