They say there are 3 mountains in the Magic Kingdom (I say 4 as of last Spring). The first time I ever went to Disney World in 1988, my favorite was Big Thunder Mountain Railroad.
Park: Magic Kingdom
Height: 40 inches
Fastpass+ Available
I was five and roller coasters were not my thing. I would ride only one - The Trailblazer at Hersheypark. It was just full of twists and spirals. I was assured this was just like that and was tugged on the ride. IT WAS AWESOME! I rode it over and over. It was the Runaway Train ride (I didn't learn its' actual name until quite a few years later.) It's a pretty long ride for a roller coaster as it takes you through the haunted mine town. I remember being 5 and marveling that the train did not have a conductor. I still love this ride today. I now love any and all roller coasters and this is still a favorite. It's been refurbished since but has only since improved it. This is a great ride for everyone. I still think it's a great first coaster for kids to try because it's not about the big drops, it's about twists and turns. I do recommend getting a Fastpass+ for this because it tends to have long lines since they can only run a certain amount of trains at a time (instead of a constant moving of cars).