They're rated AA+/AA- depending on whom. The downgrades to the US's credit rating were big news: https://en.wikipedia.org/wiki/United_States_federal_governme...

And also completely meaningless as a credit rating in the context of creditworthiness specifically means the ability to repay. And they can always print dollar bills to do so.

Now whether that $1 in 20 years will buy anything is an entirely different story.