I have a few must-dos at each park:
MK: Space Mountain, Splash Mountain, 7D Mine Train, Big Thunder Mountain Railroad, the People Mover, Pirates
Epcot: Test Track, Mission Space, Frozen but only if you have a fast pass or the wait is reasonable, allow plenty of time and appetite just to walk around and look through stores, stay for IllumiNations (my favorite nighttime show at WDW)
Studios: While I could alternate Rockin Roller Coaster and Tower of Terror for hours, I also enjoy TS Mania (Fastpass only), the Indiana Jones show, and on occasion Lights Motors Action
AK: Expedition Everest and the Safari are my only must do's. This is typically a half-day park for me. It's a very interesting place to just walk around but in terms of attractions those are it.