Both are great on immersing you into the theme, but I have to go with Splash Mtn.
As far as Carousel of Progress goes, I do agree it needs a rehab (especially the last scene) but I would be devastated if they ever permanently closed it. It is one of the few rides that has Walt's fingerprints all over it and it is a reminder of the classic old Disney. I make a point to ride it at least once if not twice every time I visit WDW.