It is subjective to our personal tastes and reactions.
It is subjective to the culture within which it is being digested, and the historical time frame. You can't tell me a play will be received exactly the same now as it was a thousand years ago.
Our gender, age, political and philosophical leanings, our religious beliefs or lack thereof... all impact on the way we digest a piece of art and/or the value of the artist.
So how can we label an artist or artwork as best? When it comes to theatre I feel as though our work is always open for reinterpretation, and to be true to our creativity we must observe the multiplicity of perspectives that any audience brings.
I once directed a piece where an audience member was peeved that I staged a scene with the actors facing a giant mirror upstage. "I don't pay to have to look at the actors in a mirror, I paid for the ticket so they should face me!" they exclaimed to one of the actors. Now, beside the fact that I think it's in poor taste to approach an actor directly after a show and criticize said show (also not helpful that the actor was related to me), and that it confuses me that someone who is in the industry would so readily jump to stomp on a directorial decision just because the actors "aren't facing" them, I was not upset in the slightest by the comment, because it's subjective. (Okay, there's also the fact that for that one person there were 30 more who took time to seek me out and mention how much they enjoyed that exact scene.)
My way was not best. It was just my interpretation. When I perform on stage and receive praise it is not because I was best, but rather that I managed to connect with and engage that audience member.
So it disheartens me when I see an arts organisation, especially those who work with young people, talking about the 'best' talent.
I teach my young actors that when they don't get a role it's not because they aren't any good and it's certainly not because someone was better than them; it is for a myriad of directorial decisions that require specific casting choices. I say that, because it is subjective. There's no measurable time, height or distance to validate a 'best' value. There's no way to create an absolute authority to defer to (other than those false gods created by elitism and good marketing which cater to the trite and stale - of course that is merely my opinion).
In short, I say we spend less time creating value classifications when it comes to any kind of art work and spend more time engaging with artists to explore from where their perspective has arisen.