the obvious answer is the first walk into the MK, but for me its the little interactions that make it magical.
We last went in 2013 when our daughter was 6. Totally amazing week, but one thing stood out
After a ride on Pirates of the Caribbean one late night towards the end of the trip, my daughter was browsing the Arribas Bros. cart nearby and fell in love with a miniature glass figurine of a seal. We had one more night in the Magic Kingdom so we wanted to wait until we bought anything so fragile - not the answer she wanted to hear. My daughter was overtired and bit cranky, but the cast member manning the cart made a point to engage her and promise that she would personally save one of the figurines for her and that if she came back the next night, she would be there and have it for her. They exchanged names - "Pirate Amanda" and "Princess L****" - and we went on our way.
The next night, we strolled by the cart but my daughter was no longer in her Bibbity Bobbity Boutique garb from the previous night, so we didn't assume "Pirate Amanda" would remember us. "Pirate Amanda" came right up to my daughter and called her by name and remembered she was looking for the seal. Totally made our day.