Ahhh, there's so many places.... I'll try to narrow it down
Magic Kingdom - I would say the castle and MSUSA, but usually, at least on the first morning there, we rush right past it in our rush to get to the rides. Later we come back, take pictures, and appreciate it, and the site of the castle as the focal point behind MSUA (picture looking out at it from the train station) is one of the best things I think I'll ever see. But the Magic Kingdom moment where it really hits me that I'm "there" is when we go down into the tunnel at Space Mountain (always our first ride there). I love how it's instantly quieter and you can hear the Star Tunnel music, and it's practically cold after the heat already building outside. And, as I said, it's always our first ride of the trip, so that adds something too. MK Honorable Mentions: the queue at Splash Mountain as you get close to loading as the anticipation builds, and the aforementioned castle/MSUSA.
EPCOT - When you walk in and see how impossibly big SSE is. Seriously, every time, it's somehow bigger than I remember. You can't get the full effect on the monorail, or even at the front gate. It's when you walk in and SEE it, rising up above you, and it just looks so huge. That image is what makes me realize - again - that this is Disney, and no one else can replicate it. Who else would think on such a big scale? EPCOT Honorable Mentions: when you see the fountain (everything great comes with a fountain), standing in a long, slow moving line at TestTrack (because even though you're stuck in line, you KNOW you're at Disney, and sometimes the best/funniest trip moments happen in line), and Illuminations, coupled with the sheer number of people who try to leave at the same time afterwards (knowing that so many people were affected by the magic, at least to some degree, is magical in and of itself).
Hollywood Studios - When you first enter and see Sunset Blvd. If I had to choose one place, this might be it. It's like you can feel the day's oppportunities piling up already. DHS Honorable Mentions: looking at the hand prints in front of the Great Movie Ride, standing in the ToT queue for the first time that trip. Heck,
riding ToT for the first time that trip (riding is cool in itself, but every time we go, my sister acts kind of standoffish at the beginning of each trip, like she thinks she's a little too old for this, and then she rides ToT and comes off and is as passionate about Disney as a five year old. It's awesome.)
Animal Kingdom - when you have to walk through a jungle before you get to anything
I guess a lot of mine are probably specific to me, and the memories I have of these places. :shrug: