I haven't been to the Epcot version yet, but I've been on the DL version of Soarin' Over California. It's a definite don't miss. Having been on most of the other don't miss rides at WDW, I'd say that I can't think of one that I'd rank above Soarin', except for the Tower of Terror, which I think may be the best conceived thrill ride I've ever been on anywhere.
MK is interesting. I actually don't think there is a true "don't miss" there, except maybe just sauntering up main street. There's no ride I've been on every time there. Maybe Dumbo if traveling with a preschooler. My favorite attraction there, when it's open, is Timekeeper.
Other than that, I'd say the only absolute under-no-circumstances can you miss them attractions in the world are the AK safari and Fantasmic. Space Mountain and Rock and Roller Coaster if you're into roller coasters.