IMHO, the very best rides combine epic scale, attention to detail, great storytelling and perhaps a few thrills. The ability to pull this off makes for a classic attraction. I'm partial to water-based rides, too, since they add a dynamic element not easily simulated.
Splash Mountain, for example, is a fifteen minute ride. It's also a physically large attraciton, yet has nice attention to detail (think of the dancing water fountains). The storytelling is very good. And the big drop is thrilling. As others have noted before, it's also a great attraction at night.
Big Thunder, Tower of Terror and Pirates (except perhaps the thrill part) all manage to combine each of these things well. I'm not a _huge_ fan of the Haunted Mansion, but I think it's a better attraction overall than Space Mountain. But then again, I'm still bitter that SM got rid of the RYCA-1 scenes at the end of the ride.
Following the same criteria, Epcot is my favorite park. What it lacks in thrills it more than makes up for in positive, can-do optimism. And sushi.
Sincerely,
Len