I haven't been to any of the other parks other than WDW ones. I have been to MK dating back to 1979/1980.
The five things I think MK does best are:
1) Give that "Disney Magic". From the first walk through the turnstiles onto Main Street, you are typically overwhelmed by the sight of the Cinderella's Castle. As we all know, the forced perspective makes that look effective. It is easy for the first time visitor to be overwhelmed by that original view of the castle.
2) Make Memories. All of us have tremendous memories of our visits to the MK park. Whether it be the rides, character encounters, or shows; something about this place captured our imagination and kept it fresh in our minds.
3) The roving Saxophone Quartet. They are often in Fantasyland, but occasionally make the trip to Main Street. They are exceptional musicians with great personalities. I was blown away that the bass sax player REMEMBERED my wife and I from our previous trip 11 months prior. That personal touch really stuck with me.
4) Their treatment of the Developmentally Disabled. My last visit was with one of my clients from the group home I run. They gave us carte blanche treatment and made his first visit so memorable, including one of the Dream Squad setting up my client pulling the sword from the stone and having him named honorary Prince of the land.
5) The rides: Let's admit it, a number of the rides hold that awesomeness that makes us want to go back again and again. Mickey's Philharmagic is a new favorite of mine, but I have lost count of the times I have ridden all of the rides at MK.