This is tough....hmmmm. Have to go with Splash. It has everything - great theming, that smell of Disney water, unbelievably catchy tune, great views and fun drop.
Great question. I love so many of the attractions. If I had to choose only one, it would be Soarin'. I feel so free and relaxed when I'm flying over everything. I think of how much my father would love it and that makes me miss him, but at the same time, close to him.
I think I would say Splash because most people have been on a 'log ride' at other parks. Splash is a great example of how Disney does theming better than anyone else.
This question makes me sad. But if I can really only have one, I have to betray Big Thunder Mountain and visit the Haunted Mansion.
I don't know what I would do if this question involved Disneyland (7 year old me loves Pirates until the end of time but Indiana Jones and Radiator Springs try really hard to knock those Pirates out of contention).