Personally I think that arguing whether her voice was "the best" (or not) is an irrelevance.
Her style is different to other female singers, some are softer, more melodic, others have a harsh "rock sound", and others are operatic.
As Carole King said herself when she recorded "Natural woman"..... it was never going to be a comparison to the power and feeling of Aretha's version, but it is in itself a brilliant track
Perhaps it is better to say that her voice was iconic, powerful, recognisable and that she was able to move audiences with it