I could write a term paper on this topic.
The Haunted Mansion is the quintessential Disney dark ride. For starters, the outside sets the tone for the ride. Ghostly shapes fly past the windows of the old dilapidated house, as wolves howl and thunder claps in the background. Yet when you get up close to the Mansion, and see the tombstones laid out in front you realize that this will be not only a ride filled with 999 happy haunts, but a bit of a laugh as well.
The preshow also is something that isn't done anymore, a preshow with physical sets instead of screens. The stretch room is so iconic.
Then the ride. An AA filled masterpiece with some of WED's best effects work. There is nothing like it, including Grim Grinning Ghosts. It's a masterpiece on the highest level, the only thing at MK that comes close to touching it is the Jungle Cruise.