Has to be MK, first and last. There is something about seeing the castle your first day/night there, and then we always have to see wishes our last night there. I feel like it would be weird to do anything else because it has just become a special part of our visits. As a 25 year old man I still get an awesome feeling the first time you walk down Main Street at the start of a trip!
Well Actual I don't have a specific park that I do on the first day... usually I do whatever park has EMH that first day. If I had to have park it would have to be the MK as its a great way to start your magical vacation where the magic all began for WDW. I'm no WDW ritual type person more of whatever I'm in the mood for type person.