Wow, that's surprisingly tough!
If I could only pick one, it would have to encapsulate all that I expect from a WDW attraction. This may strike some as a strange choice, since it's not a classic, but I'll have to say...
Mickey's Philharmagic
...At least for now. I'll probably mull this over when I go to bed (or even whether I could actually narrow it down to just one). I think I'm waaay over-analyzing this - time for bed!