Big Thunder Mountain (my DD's favorite and our ride that we HAVE to hit as often as we can while we're there) and Dumbo (it's such a classic and a tradition)
If I am allowed to count The Land pavilion as one entity, I would say that.
But to chose one specific attraction/ride, it's close between like ten different things, but I have to say Big Thunder Mountain. Honorable mention to Test Track, Spaceship Earth, The Little Mermaid, and the Peoplemover.
As a history teacher, I love Carousel of Progress
But my favorite attraction is Tower of Terror. I love the theme. The time period that it is set in is my favorite. Im a huge fan of 1930s and 40s Hollywood/LA. Plus the story is awesome
To pick just one is a disservice to so many others. Each of them are favorites for different reasons. But the one I have to visit every trip and multiple times during the trip us Haunted Mansion. Even the upgrades added were true upgrades. That can't be said of many attractions.