The worst is definitely AK. There's not a lot to do inside. Dinosaur's inside queue isn't too long (not an issue in the offseason), Everest has a lot of outside queue, and the only continuously running show is It's Tough to be a Bug. Everything else is either showing only a couple times a day or is outside, off the top of my head.
As for the best, there's an argument to be made. I'd say that Epcot is pretty decent, given that there's a lot you can do to eat a lot of time during a rainstorm. More often than not, you can get in line for Spaceship Earth when it's raining, and by the time you get out, it's beautiful outside again. Other than Test Track, pretty much everything is primarily indoors. Innoventions is another good indoor time killer. World Showcase has a lot of shops to pop in and out of, as well. Epcot's main disadvantage is that it's very spaced out. Getting from one pavilion to another takes a while.
And that brings me to Magic Kingdom. Again, a fair amount to do inside, plus a few continuously running 15-20 minute shows, and the park is set up without the space that Epcot has.
Hollywood Studios isn't awful in the rain, as it also has a few attractions that can kill a lot of time, and pretty much everything is indoors. I feel like it's spaced out a little more than Magic Kingdom is, though.
I'd say Magic Kingdom, but Epcot is a close second.