UO: Shrek. 'Nuff said.
IoA: This is a really hard one. I know most people are going to say Storm Force Accelerator or the drop tower. I'm going to say Jurassic Park. I'm a huge fan of the movie and the ride just doesn't do it justice. It's so close, but so, so very far. Plus, it's a totally unnecessary water ride for the park.
MK: Stitch. It seems like it has so much potential

HS: Backlot Tour. Such a dated waste.
(Haven't been to Epcot or AK in a really long time!)