Haunted Mansion!
I have yet to ride the new Mine Train, and it probably is better at night, but I don't see how any attraction could ever top the HM.
I love the parks at night! There are many rides that are different under night lighting. They are not always better, but different. Lighting improves some aspects, and diminishes others. The castle is a great example. I love how it looks at night, but the daylight castle is the first view most guests see. that's often the one that sticks in one's mind.
Beyond HM, my top picks are: PotC, IaSW, Astro/A's Carpets. See WISHES! The night parade at MK blows away all the rest. (except the cute water parade, which is also at night.)
At HS, RnRC, TSM, ToT.
At Epcot, walking the WS. None of the rides really stand out, but walking the WS is special.
AK is rarely open after dark. Mostly, I've only seen it as we were basically headed out of the park. Late safari rides are good, but that's usually at like 5:30pm. EE late was pretty neat. Just walking the pathways is pretty fun.
After dark, we often eat dinner at AKL. It is hard to see the animals at night unless a CM gives you a pair of night vision goggles, but those are really fun to try!