What makes a good fly line? Probably depends on whom you ask. Besides casting whatever you want it to cast well with the rod of your choice, there’s other considerations.
How does it handle different temperatures. Do you fish off hot sunny boat decks or wading in the shade? Do you fish in the cooler weather season?
Is the line prone to getting dirty? Is it prone to tangles in the running line? Is it too slick making it difficult to set the hook?
My 8 weight SA sharkwave Titan taper line casts well, but gets extremely slick in hot conditions. So slick I have an awful time setting the hook and keeping pressure on a fish.
My Rio Outbound 8 weight line is very prone to absorbing marsh mud and getting it embedded in the coating. Eventually, it affects the handling and performance and no amount of cleaning will remove the particulates.
My Airflo tropical punch stiffens up in cooler weather and becomes hard to handle. On the other side, I had some cabelas line that got limp in hot weather.