I would say either Epcot for FutureWorld or whatever it’s called these days and Mexico but not for the rest of World Showcase except for the indoor movies/museum exhibits. I wouldn’t want it to be too rainy on a day I planned to catch a lot of outdoor live entertainment though and it would affect Test Track potentially. Hollywood Studios would be my second choice since only Slinky Dog Dash would close.
Animal Kingdom would be the worst since most of its outdoors. Magic Kingdom there are four great/classic outdoor rides and then the kiddie rides like the Dumbo circus area and Astro Orbiter and Magic Carpets could all get closed or be drenching. Plus Magic Kingdom also has outdoor things like fireworks and parades so it’d be a close 2nd to AK for me as worst park.