Big Thunder at night is my single favourite ride.
We rode Incredicoaster at night a bunch this last trip and that is a lot of fun.
Splash Mountain as well.
Basically, I like rides where you can see clumps of light/colour all throughout the park. The lights along the Rivers of America, the Pixar Pier, the Radiator Springs backdrop, that sort of thing. It's really beautiful. Add the night sky too.