I love it when it rains, so long as there isn't any thunder and lightning in the mix.
In Epcot, you can still do just about everything, even with thunder and lightning. The beauty of Epcot (especially Future World) is that the park is divided into buildings, and within these buildings, there is a lot of stuff to do. You could spend a lot of time in Innoventions. The Test Track post show has a lot to look at. The Land has a ride, a show, a shop, and food. The list goes on and on. The only bad thing is that the entrances to these buildings are somewhat far apart (since the buildings are so large)
The MK in the rain is a blast, because everybody ducks into shops and restaurants, and you can go to rides with little to no wait. MGM is similar.
The only park that stinks in the rain is AK, because all of the animals go into hiding, and there's not much else to do except hang out in Dinoland or go on Kali River Rapids. However, as soon as the rain stops, the animals come back out because it's nice and cool for a little while at least.