E, we usually head to Epcot when the forecast calls for rain all day. Most of the Future World attractions are indoors and it's fairly easy to spend a lot of time enjoying those attractions. In the World Showcase, the pavillions are pretty close together, so if you sprint from building to building, you won't get too wet.
We tend to avoid AK and Downtown Disney on rainy days. AK because almost everything is outside and DD because that's where the masses seem to go.