I had to vote EPCOT. The bold lighting and the feeling of building excitement leading up to Illuminations gives an energy that's hard to beat. The World Showcase is very beautiful at night, and I think the individual countries seem even more realistic.
The Magic Kingdom is a very close second. To me Magic Kingdom is a bit different in that I like visiting at night because many of the attractions are more effective at night, as opposed to me finding the actual park more attractive. While I do like the subtle use of light and color, I really think the Magic Kingdom's real beauty is best shown during the day. But the Jungle Cruise, Big Thunder Mountain, and the Carousel are all much more fun when riding at night.
Animal Kingdom is third. I've only been there after dark once or twice, but the park really takes on a very different look and feel at night. You feel much more isolated, and the feeling of really being in a remote, natural setting is heightened significantly.