I would say Big Thunder Mountain. It is my fav!(especially sitting in the back) With Splash, it is a hit or miss whether you'll get pretty wet or not. Sometimes, those water cannons aren't friendly! But, I love it too, I just prefer BTM.
I prefer the original version of the Living Seas attraction, the version that didn't have the Clamobiles and instead had the hydrolators and seacabs.
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The following image is my own rendering of a ride vehicle that I believe would make the ride better. To learn more about it, visit the Disney Rumors and News forum and click on "My Version of the Ride Vehicle for Epcot's Nemo Ride".
Splash Mountain without a doubt. BTMR was boring, in my opinion, when compared to other coasters. The theme is great, as it is with most other attractions, but SM is a lot more entertaining thrill-wise. Plus it's longer.