AAA — S&P / Fitch
Aaa — Moody’s
AAA/Aaa rated bond issues have an almost nonexistent chance of defaulting, according to the major ratings institutions that issue the ratings.
AAA/Aaa is the highest rating a bond issue or company can get.
In the aftermath of the 2008 financial crisis and recession, many companies, and the US Government itself, were downgraded from AAA to AA+. Only two companies in the US still retain the AAA rating: Johnson & Johnson and Microsoft.
Bonds can still receive a AAA rating even if the company issuing them does not have that rating, due to the fact that they can be backed by surety insurance contracts and hard assets.
S&P, Fitch, and Moody’s are the Big Three ratings institutions, and while they do not always give the same ratings, they are generally in-line with one another.