I love lots of rides at the Disney parks, but I have to say Splash Mountain - no contest.
It may be a newer ride, it may still be operation, it may be a "thrill ride," and it may be popular (these all seem like negatives on this board sometimes), but the best ride I've ever been on anywhere. Great, immersive themeing (the tree leaves completely masking the ceiling!), great story, immense fun from start-to-finish, with surprises (who'd ever been on a flume with that many drops?), fun, thrills, and a great story. It's appropriate for just about anyone over the age of four. Go on it with people who never have before, and are used to amusement park flumes. You'll see the pure enchantment on their faces. On top of it all, it's a LONG ride!
Sheer genius and pure Disney magic.