I think it's safe to say Rio del Tiempo is probably the most underrated ride of all time.
Having said that, Malestrom blows it out of the water. Rio's temple at the beginning is super sweet, and some of the video (the dancers in particular) are funny in a cheesy sort of way, there's never a line, and it's a super-nice refuge from the heat.
Even with all that going for it, Malestrom is a lot more fun. The trolls, shooting down the flume backwards, almost going over the waterfall and of course, skipping the movie at the end add up for a fun little experience that, along with Rio del Tiempo, help mix up the World Showcase... I kind of wish World Showcase had more lower-scale rides like these for other countries...