This might sound strange, but my first ride on Space Mountain of a trip always gets to me. I get overwhelmed with a crazy happy content feeling! I think it's because it's the first indication that I'm actually in Disneyland (since for as long as I can remember it's usually the first ride we go on the first morning of a DL trip).
Other than that the usuals: Parades, Fantasmic (esp: the princess part & the end), the fireworks (esp: the beginning part of Remember with the characters all telling their wishes & LeAnn Rimes after the fireworks). The worst one though is always the snow after the firework during the holidays. It always gets to me especailly that version of White Christmas. That is the only time I let myself openly/visibly cry in the park