Not technically a ride, but hands down Carousel of Progress. We got stuck in it and had to watch the same scene 3 times, and I think another scene twice too. It may have even been more than that. It seems like we were in the thing for an hour. Seems like we were stuck halfway between scenes for a while too. I think they were "rebooting" us, but then it would glitch or stick again. That was several years ago, and we haven't set foot in it since! We felt trapped!
We've also been stuck on Small World for 20 minutes, but right as you are moving into the last area with the signs so not much to look at and that song going over and over and over. We've also been stuck on the Monorail for 20 to 25 minutes, and it gets hot and also a little nerve racking being up high on a rail not moving for that long. Funny, there are multiple posts about being stuck on HM right where the ghost pops out from behind the gravestone- we've been stuck in that spot too- probably not more than 5 minutes- but I still didn't like it!