Usually I'd say PotC or Splash, but this year Pirates will be down while we're there and I'm pregnant (so no Splash for me). Luckily I'll be able to live vicariously through DD who's planning on riding for the first time! Our 'can't miss' show is IllumiNations.
There are so many rides and other things that we'd be sad to miss (Haunted Mansion, Jungle Cruise, ToT, the safari, World Showcase,...). We'd never be able to do just a day or two trip to WDW. I think if we DID have just one day at the parks, no matter what rides we went on or shows we saw, we'd probably make a point to get a family picture in front of the castle.