Today's Writer's Almanac quotes former New Yorker poetry editor Howard Moss as saying a good poem is "one I like." Although I would disagree with him on quite a few of those poems (I rarely like the poems printed in the New Yorker), I have to admire such a bald statement on the impossibility of objectivity.
True, one can strive towards objectivity - like my current MFA advisor, who recommended I read and learn from Jean Valentine's work even though she doesn't like it. (We'll see what she thinks of what I do when inspired by a poet she doesn't care for!)
It's also true that one's opinion will carry more weight when it's backed by the experience and breadth of knowledge that comes from years of work and study, as one would presume in the case of Howard Moss - but the fact remains that value judgements are always subjective, and there's no objective criteria for evaluating art.
I'm aware that this is a philosophical position and that some may disagree. For hundreds, if not thousands, of years the so-called "objective" standards for evaluating art have gradually evolved, but usually come down to the consensus of a large enough group of influential people.
An article published by the Poetry Foundation about high school students' reactions to canonical poems gives a refreshing take on the old standards. "Herbert sucks. Donne is a pimp." The high school students did trash my beloved Elizabeth Bishop, but at least they did it in an entertaining way.