What makes a great brush is better answered - what do you need a brush to do and that is better answered by what is your story? A brush is nothing more than a tool, but as Picasso said of Monet, "Monet was just an eye, but my God, what an eye."
So, I guess that means a brush is important, but what about the eye part?
There are actually two parts to this that have to be addressed:
That you've developed an eye with your hand and your touch to be able to see the subtle changes and variations that a brush can make and do for you. Without that, we don't have to worry about the rest of this talk.
If you're unaware of what you need the brush to do for you in that painting, in that given spot, at that given moment. Without that, we don't have to worry about the rest of this talk.
We become Alice in Wonderland at a fork in the road when she asked the Cheshire cat, "which way to go?" The Cat responded, "where are you headed?" Alice responded, "It doesn't matter." The cat responded, "then, it doesn't matter which way you go."
At this moment, Alice has four choices:
She can turn and go back.
She can stay right where she's at and not progress at all in any direction.
She can just guess and go either direction or hope it's correct if there is a correct.
She can listen to an old art teacher, who has all the bumps and lumps and battle wounds from failing in art for a lifetime, suggest listening to this quiet voice in the back of her head, saying, learn to draw, learn to shade, learn to develop your touch, learn all the drawing concepts you can find because you are trying to tell a story.
If you don't know the story, then you don't know which brush to use, let alone which medium to use with each brush stroke.
So, somehow, this Brush topic has now turned into you telling a story, your story, in your art. Art history is dotted with artists who have discovered different ways to perceive the world around us and to convey it on a surface for others to enjoy.
It seems easy to come up with a solution that says shouldn't we know the past? Understand the foundational core drawing and painting concepts, and allow them to be tools in your art today - tools, not rules...Alice?