Actually I had incredibly different tastes as a kid, and there is a lot of stuff that just wasn't interested to ride at the time, though as I have grown older I enjoy and appreciate just about every ride there. In Magic Kingdom at least, as a kid you could leave me in Tomorrowland all day and I would just be the happiest ever.
As years past, and in my mid 20's now and finally going back to the parks, it boggles my mind how many rides I didn't ride as a kid, either out of disinterest or simply not knowing they were there. As luck would have it though I did have the foresight to ride on just about every big ride that was eventually removed or replaced. I only rode 20K once, but that makes it more special.
But, to answer the question....probably Big Thunder Mountain Road. While both it and SM are on a similar level of "thrill" it was just lacking the same theming and atmosphere that makes me drool over SM so much, even to this day.