That's really a hard question. Have you been to Disney before or is this your first trip.
Either way, my must not attractions are:
Everything in Magic Kingdom
Epcot: Soarin, Test Track, Mission Space, World Showcase, Spaceship Earth
MGM: ToT, Rockin Rollercoaster, Fantasmic, Startours, Muppets 3D, Millionaire
Animal Kingdom: Expidition Everest, Safari, Dinosaur
There are lots more but, these are probably the most popular attractions at each park.
But, if you have little ones, these may not be the ones that you need to focus on!
You'll have fun no matter what you do!