If I could only do one thing at WDW, it would be Country Bears. When that gets changed, it would probably be Tiki Room. As far as current rides, PotC at MK. ToT at HS. EE at AK and I guess Soarin' at Epcot.
My favorite thing ever in WDW was the old good morning show at MK on the train station platform. There is so much excitement to get into the park. And then you walk under the tunnel and MK is revealed all at once. I miss that.