The correct answer is: AM Best is one of the better-known rating agencies for insurance carriers, but is by no means the only one or the most accurate. The big three credit rating agencies, in no particular order, are Fitch Ratings (Fitch), Moody's and Standard & Poor's. These agencies also provide credit ratings of insurance carriers.
The title of my answer is Insurance Carrier Financial Ratings
My answer's reference is Connor & Gallagher