 There's a lot of different agencies that rate bonds. One is Moody's, another one is Standard & Poor's, and they really focus on the credit worthiness of the bond. Like, what's the odds of this bond defaulting and you not getting your money back, in other words? So the highest credit score for borrowers, be they companies or countries, for that matter, is AAA. Both agencies that I mentioned use the same designation when it comes to the very best, most reliable debtors, AAA.