Well, this is my favorite park (which is a surprise to people) and to me there are alot of not to miss shows and rides. They probably will vary between person, but here are some that I can't miss:
Mission Space (Great Ride)
Test Track (One of the best on Property)
Ice Station Cool (Free Drinks)
Body Wars (Old, but always a short ride and I always think I'm going on Star Tours in MGM, the ride type is the same)
Living W/ the Land
Journey Into Your Imagination (I find figment funny since I was five, that might just be me)
El Rio del Tiempo (If there is a long line I would skip, it's not a thrilling ride)
Maelstrom
Living Sea
O Canada (Like most of the movies in the World Showcase, you just need to see those once)
Universe of Energy
There a few that you could live with missing, but out of those I would have to do Mission Space, Test Track, Universe of Energy, and Living W/ the Land. Have Fun