[Amneris]

I think Jungle Cruise must win as they'd have to do all the excavation for that very early on in the WDW construction process. (Piling up the land to build the Castle on was problaby THE first thing done, but the Castle isn't a ride so can't count.) About half of the current MK rides must have been there when the park opened, I think. As for the idea for the rides, many came from Disneyland, and the World's Fair inspired rides.