I'd say the Mad Tea Party. During our last trip, our nephew and his friend (both 19-20 yrs old) kept telling us they were NOT riding that "kiddie ride". Well, we finally got our nephew to ride it with us, and he laughed the entire time and even turned green-- yes, literally! Needless to say when he got off the ride his whole outlook had changed!
The Timekeeper Time Machine attraction with Robin Williams as the robot. The only problem is that you have to stand the entire time. My kids really like Alladin's Magic Carpet ride. It's the same thing as Dumbo without the awful wait.
Carousel of Progress, especially since I hear they are going to close it down. I love having new attractions come in but not at the expense of a Walt Disney classic!
For us, its COP and peoplemover. Been there since beginning and are two of the best. Pirates of Carribean is a sleeper too. Not too many people knew it was back in there years ago but with the movie, everyone rides it and seeks it out!
Carousel of Progress. Instead of destroying it, I wish they'd just move it to EPCOT.
TTA as well.
I can't agree with Peter Pan. We have some very funny pictures of us FINALLY riding it (after having 2+ hour waits when Splash Mtn was a walk-on). I think we finally got on it at 2 AM.
I agree with the TTA but I have another couple of choices. I love riding Winnie the Pooh's Honey pot and the Grand Prix Raceway. They both let you sit down for a while and it let's you remember what it was like when you were a kid when you watch your own kids drive.