I agree that only AK will be difficult in the rain, but we choose Epcot when it rains. There are more indoor dark attractions that can hold a lot of people and take a long time, such as Universe of Energy, The American Adventure, and all the movies (O' Canada, Impressions of France, et al). Spaceship Earth is a longer ride, and there's so much indoor country exploring. So for us it's the park with the most out of the rain options, and the most capacity to keep the indoor areas from becoming a zoo on days when it rains all day, rather than a shower here or there.
Pat