This question is really so subjective. Plus, all the most popular shots have been so done, they are almost cliche. Yes, a picture in front of the castle is nice, and it is a "must have".. but it's also unoriginal and cliche.
So my advice is simple -- Capture the fun and essence of YOUR experience. Capture your favorite things at Disney, as well as the overall flavor of your own vacation. Don't forget to take lots of candids. Get pictures with characters, if the characters have meaning to you. Capture scenes that will refresh your own memory of your experience and vacation. Use your camera to tell the story of your vacation. Whether it be the fireworks or parades or the castle.
I'm not always as good as I preach. I end up with way too many posed pictures and not enough candids.
But that's the must have --- The look of joy and excitement captured on your 3 year-olds face, during some Disney moment.
I have a picture from my first Disney trip -- My 2 kids walking hand in hand ahead of us towards the castle.
Disney 2011 best-79 by
Havoc315, on Flickr
This is my own person must-have shot. Couldn't have planned it.