What is the one attraction you would love to do again, but can't! I'm thinking primarily about WDW attractions from the past that have been maliciously and evilly closed/retired/destroyed. You could, however, go with something that you can't do for some other reason, like physical limitations.
My choice is pretty easy...Horizons from what was then Epcot Center. That was my favorite ride there in the 80s. I loved everything about it, but my favorite parts were the film sequence on the largest movie screen in the world (especially the Space Shuttle launch!), blasts of oranges from the desert, and the voted-upon ending. While the final simulation sequence wasn't the most exciting thing around, it was the precursor of what was to come over the Disney Horizon. (Sorry, I couldn't resist.)