Relativity gets my vote.
While I'm not terribly well-versed in Egyptian history and architecture, the pyramid is a simple and stable shape, especially one with a wide base relative to its height. The pyramids are an incredible achievement, but to me, it is more an achievement of organization and logistics than an intellectual one. They took a familiar shape and scaled it up to monolithic proportions, but as far as I know, didn't take advantage of more "intellectual" inspirations in architecture like the arch.
Relativity is the product of the type of mind that (like Ronda Rousey) appears almost "once ever." Thinkers like Pythagoras and Issac Newton and Galileo and Albert Einstein get to places where few humans in the history of existence were capable of making that jump. What comes for centuries is built upon their inspiration.
The arch was used by the Ancient Romans. I'm more intellectually impressed that they incorporated that theory into their buildings at such an early time.