Which piece of antique would you consider most valuable in Western civilization?

Well, the price of an antique, like of anything else, is simply the maximum amount of money someone is willing to pay for it. The most famous antiques (so probably also the ones with the most stories attached to them) are owned by museums who aren't selling them at any point.

So, at how much is the Mona Lisa valued? It's priceless, because there is only one, and the Louvre is not selling it for anything - so its value is infinite. Many artifacts like that (the Cathedral of Chartres? The Magna Carta? The AmericanDeclaration of Independence? etc, etc.) are similarly priceless, and it's not possible to judge which is *more* priceless.

Still, as a thought experiment, who would be willing to pay the most money for an artifact? Maybe it is Steve Wynn, a Las Vegas casino owner, who paid $105 million for Picasso's 'Boy with a pipe', if you consider Picasso to be antique enough. Paying that much for a not-so-famous-before painting, we can expect the way, way more famous and significant Guernica to fetch several billion dollars, if it was on sale.

The most interesting and commonly over sighted version of the Declaration of Independence is the first: Jefferson's original four page rough draft.

Definitely this would make it the most valuable antique in western civilization to me.

If you can get away with chips or even a huge chunk from the Wailing Wall, then you have a big piece of Western civilization/history there.

Or try getting some of the stones from the entire Greco-Roman ruins in Greek and Italy, respectively.

Anything related to when the white man came to America and the guns from civil war and clothing, blankets from the various Indians from 200 years ago. All these and anything Chippendale or Tiffany. If none of these work, try Star Trek and Cabbage Patch doll collecting...LOL

Apparently its a 1904 Honus Wagner baseball card.

anything related to the era.