Oddly enough - the Pyramids of Giza. I guess that's because they are amazing in context. Huge, accurate, heavy - and they were built first - that is before anyone else built anything like a permanent structure. The structural size / useable size ratio boggles the mind - so much stone for so little space - and yet that fact itself is amazing, how did they keep the innerspace on track in the construction. On one hand they are a pile of rocks - nothing special, but I drive by them every day and see them in every light, in fog and in cloud, at dusk and at noon, and they amaze me everyday. Permanence is a beautiful thing
How many of the seven wonders of the ancient world can you name without going to reference?