It really depends, usually when we go we check what days the parks close later (for example, if Epcot closes at 9 both Saturday and Sunday, and Hollywood studios closes at midnight on Saturday then we go to HS first). We always base our decision as to where we are going according to that, trying to get the most time in every park .
We always start with MK regardless of crowd calendars - it's not completely "Disney" until we see the castle and smell the popcorn!
I vote none of the above, there isn't an always clear answer. I think it depends more on arrival time, half day or full day, park hours, special events going on, EMH's and more.
For me it's EPCOT at least for the majority of my 8 trips.
Here's the reason: Even with leaving on the earliest flight from PIT, usually around 7AM, and checking in (and eating lunch) at the resort it is always sometime around noon before we get to the first park.
This only gives us about a half day there, so we usually do Future World and possibly some of World Showcase before it closes. Most of the time at the end of the week we go back to World Showcase and possibly park hop.
Always visit MK first. There's something about entering the park and going around that first corner where you see the castle in her full glory. It's just magical and sets the tone for the whole trip. We then do HS, EPCOT, AK in that order or switch it up depending on crowd levels.
I voted for AK since we always go to a park on arrival day and it's our least favorite so we need the least amount of time there.
Magic kingdom!!!!!!!
No doubt---- Magic Kingdom! Nothing like the first glimpse of the castle!