Time to go against the grain, but one attraction at Disney World that I could ride all day, were it not for all the steps and the unfortunate pruning and sunburning of my body is Crush n' Gusher. I must have done that thing 8 or so times while I was at Typhoon Lagoon...and I was only in the Park for 3 hours. I'm not sure I've ever laughed so hard. It was just too much fun.
But, in the 4 major Parks, I'll go with Horizons, Irons' Spaceship Earth, or the original Journey Into Imagination. Of course, if I have to pick something that Disney didn't kill, then I'd probably go with either Muppet-Vision 3-D or The Great Movie Ride.