The USD is considered the most sound currency in the world and has been for generations; maybe it's the most sound in the history of humanity (possibly a consequence of modern international relations and communications). The great majority of international transactions are in USD. At small shops in a lot out of the way places, they accept USD; many prefer it to their local currency.

It would be a lot sounder if it wasn't inflated away by deficit spending.

"Most sound" is relative. The other countries are simply worse at inflating the money supply.