LOL! I used to be so proud of the fact that I had all my "favorite" bathrooms around the parks (mostly those that are off the beaten path, and are clean and functional). A few of my best ones have been made unavailable due to construction, but I will give you only one of my favorites in each park (but is it the best one? You will never know).
DHS - when you walk in, after security, but before the turnstiles, go to the extreme left and walk along the building. A bank of bathrooms, clean, but older (like, you actually have to flush these yourself!), and no one is there!
Epcot - Morocco, in World Showcase. Bonus - Future World, cross from Spaceship Earth's ride exit.
AK - After you exit the gift shop in Dinoland, with Primeval Whirl right in front of you, go to the right along the building.
MK - Walk past Carousel of Progress, going toward Space Mountain. Huge bank of bathrooms here.