The best glass to be had is made by Schott, and despite common "knowledge"..Schott has plants all over the planet.
Pennsylvania has a Schott optic glass plant, for example, where Vortex gets its glass for the Razor AMG.
Zeiss is also right up there, but Zeiss isn't just a company..it's a foundation, with many brands under its umbrella. Leica labs does a lot of R&D work for Zeiss optics glass as well. It's mostly made in Europe, but does have plants elsewhere. They keep their info a little closer to the chest.
China..again dispite common "knowledge" otherwise, is more than capable of making top end optic glass that rivals Schott. Any compaany who refuses to tell you, or makes you work at finding out, where their glass comes from..it's because it does, in fact, come from China. Leupold buys glass from China, so does Burris, Vortex,..everyone.
If a company tells you were the glass is from..it's for a reason, and it's always glass from Europe...or sometimes Shott. That's a marketing advantage for them.
Japan still makes optic glass, just not much for the sports optic market, and believe it or not...it's not what it used to be.
The Phillipines makes decent glass, middle of the road stuff. Most of Vortex and old Nikon sourced from there...but they don't make much, if any, high grade stuff.
The USA, sadly, makes very little. SCHOTT plant, and few other small ones here and there. Nothing major.
So really, it doesn't matter much for brand in the 500 buck range. I'd stick to the well know brands..Leupold, Vortex, Burris..you can't go wrong with any of those...
They are all close to the same in that price range...
When you cross the 2k range is when ratings get very interesting.