But I think the only really quantifiable things in regards to art are:
1. Its physical properties, which are irrelevant to its artistic merit.
2. The fact that some piece of art is popular or not with people into that type of art.
So basically, a great piece of art is any piece of art (book, painting, movie, TV show, videogame, comic book, etc) that many people who are into that kind of art generally consider great. Casablanca is a movie that many people into movies love, so its a great movie.
Things don't have to be absolutely "quantifiable" in the sense of being numeric and precise. There are community standards which we can understand, in principle, even if they're hard to apply, in practice. We can discuss composition, shot length, theme, acting quality, blocking, SFX, lighting set-ups, etc., relative to these standards and have fruitful discussions to the extent that these allow us to arrive at an agreeable conclusion.
Are there cosmic standards of art? Would reasonable Gribnars of planet Krathag be forced to admit Casablanca is a classic? Maybe not. But we haven't met them yet. That stated, we can look for aesthetic preferences which are biologically grounded (e.g., the preference for symmetry, the preference for some musical chords over others, stories that speak to the human condition), and those can be investigated scientifically. We can also look to community standards, and relative to a community judge an artwork by those standards. We can talk to each other and see what standards we share in common and use those local commitments to arrive at local conclusions (e.g., our local assessment of the quality of Casablanca). Finally, we can attempt to work from our own standards if we minimally wish to justify an evaluation to ourselves.
I think we need to resist the temptation to cut the Gordian Knot and cash out for a "TomaYtoH/TomAHto" relativism (i.e., let's call the whole thing off) in which no one is really right or really wrong. In a cosmic sense, it is likely that no one really is, but this does not mean that cannot strive to have productive conversations, arrive at temporary closures and pronounce local conclusions. There are all sorts of things that we may appeal to as relatively externalized guardrails. The pleasure of aesthetic conversation is taking an intellectual stroll with others and work our way along them and towards them (allowing our conversation not to entirely dive off the cliff into the abyss of absolute subjectivism).