|The Magic Kingdom, probably during Extra Magic Hours|
Many are decrying this move, accusing Disney of being motivated by financial reasons to save on operational costs. Those people are right.
As an "intern" in the Disney college program, I worked many an Extra Magic Hour (or Extra Tragic Hour, as we called them) at the Magic Kingdom, which often lasted until 3 AM. For the most part, I did very little work, the hardest thing was staying awake. I often asked myself "What is the point of me being here? No one else is."
I even attempted to attend the Magic Kingdom's Extra Magic Hours when my girlfriend visited, and only stayed until 1 AM, a third of the allotted time. We didn't need much more as we were tired out from what was already a long, productive day.
I did Extra Magic Hours at Epcot one night, and it saved me absolutely no time. The lines were still gigantic an hour in to Extra Magic Hours.
From my limited experience with Extra Magic Hours at Hollywood Studios, Animal Kingdom and Epcot, and my extensive experience with them at the Magic Kingdom, I feel evening Extra Magic Hours are not worth it. People will be better off with one less hour.
The opportunity to stay three hours after park closing is a hard one to give up when you spend thousands of dollars on a vacation. The extra time you're getting in the park with no lines gives you a sense that you're getting your dollar's worth. But what about the sleep you're losing that night, the fatigue you'll have the next day and likely the rest of the vacation?
The way to really get you're value is to take advantage of the morning Extra Magic Hours. You're walking into an empty park, unlike in the evening, where you're staying in a park that's already populated, where lines have to die down, and you have to stay up extra late for them to die down. Yes, the AM Extra Magic Hours are only an hour long, but guests generally don't show up until a couple of hours after the park opens to the general population anyways, so in a way, they are equal to the evening Extra Magic Hours. The lines are much shorter, and you're just starting your day out, instead of elongating an already tiring day.
It all depends on if you're a morning person or night owl, but I've seen what the Magic Kingdom, the most popular park at Walt Disney World, looks like at 2:30 AM, and you weren't there. Neither was anyone else, really. And the people who were there either looked miserable themselves, or were dragging around powerless, miserable children.