ummm... tough choice. There are so many outstanding creative attractions at WDW.
If I had to choose, though, my top three would be:
Thunder Mountain (if you haven't noticed yet, but there are so many details, it's amazing!)
Splash Mountain (again, full of detail, great music, and just awesome how they made those great stories into a ride)
Rock 'n' Roller Coaster (the way the match the ride with the music is unbelievable! They had someone go on the ride with a laptop to determine when different parts of the songs would play at different times of the ride. For example, in the lyrics of "Love In An Elevator" they say "livin' it up when I'm goin' down. Well, the imagineers made it so when they say "livin' it up", you are going up, and when they said "when I goin' down", you are going down. Just amazing!)