Festival of the Lion King. I cannot imagine going to WDW and skipping this. It moves me to tears most of the time.
OK, now that I've answered the OP's question, here's the list of must-do, absolutely can't miss, wouldn't be a trip to WDW without them, attractions/experiences:
MK: BTMRR, HM, Peter Pan, People Mover, Wishes. DD would add Splash and Space Mountains, but I don't ride them.
Epcot: Illuminations, but only with DD. I love Epcot, but there's nothing here that I would miss if we didn't do it. SE used to be my FAVORITE, right up there with Lion King, but that was before they dumbed it down and put in the foolish interactive screen to write your own story. Ruined the whole thing for me.
DHS: Beauty and the Beast, Great Movie Ride. DD would add Tot, RnRC.
AK: Festival of the Lion King, Safari. I SO wish they hadn't closed Tarzan Rocks.
Back in the "old days," when we traveled to WDW with my sister and her kids and we all stayed offsite together, it was a family tradition to acknowledge when we were on Disney property. We'd be driving towards a park and someone would spy the Disney World banner that stretches over the roadway, and they'd start: "We're in Florida... boring, boring old Florida." Others would join in- "Poor us, stuck in boring, BORING Florida," and as we passed under the sign, we'd all yell... "But now, we're in DISNEY WORLD!!" The first time DH or my ex-BIL (because the husbands don't always go on every trip) or other family members were in the car, we garnered odd looks, for sure. DD22 and I still do it (yes, we get odd looks on DME, but we have to be sitting up front to see the sign approaching) and i know my sister and her kids still do it. It's just our little family thing, I guess!