Based on the Rating Bands that we just saw, the ratings from the respective agencies are:

Prime Investment Grade: No Risk of Default

Rating AgencyRatingS&PAAAMoodyAaaFitchAAA

High Investment Grade:

Rating AgencyRatingS&PAA+, AA, AA-MoodyAa1, Aa2, Aa3FitchAA+, AA, AA-

Upper Medium Grade:

Rating AgencyRatingS&PA+, AMoodyA1, A2FitchA+, A

Lower Medium Grade:

Rating AgencyRatingS&PA-, BBB+, BBB, BBB-MoodyA3, Baa1, Baa2, Baa3FitchA-, BBB+, BBB, BBB-

Non Investment Grade/Speculative:

Rating AgencyRatingS&PBB+, BB, BB-MoodyBa1, Ba2, Ba3FitchBB+, BB, BB-

Highly Speculative:

Rating AgencyRatingS&PB+, B, B-MoodyB1, B2, B3FitchB+, B, B-

Extremely Risky:

Rating AgencyRatingS&PCCC+, CCC, CCC-, CC, CMoodyCaa1, Caa2, Caa3, CaFitchCCC

In Default:

Rating AgencyRatingS&PDMoodyCFitchDDD, DD, D

As you can see, AAA is the best and D is probably the worst possible credit rating you can get