The 'very best' is all just your opinion.
Your absolutely right.
With so much music out there, I just have to draw the line. If an artist isn't unique it tends to slide under my radar.
But, when I say the very best, it is based on history, influence, talent and a general consensus.
For example, R&B, taking into account my personal taste, I lean towards Otis, Mayfield, Marvin, Aretha, Al Green, the Commodores etc. A slightly newer addition would be Alicia Keyes. These particular artists stand out above a rather large pack.
I could delve deeper into any particular genre, looking for the little nuances that separate one band from the next, but it ultimately leaves me disappointed having listened to a lot of 'sameness'.
A truly unique artist doesn't let me make this judgement as their ability is immediately recognizable.