Even though there is no single, objective answer to this question, in my opinion one of the most important criteria is depth. Depth basically means the perfume lets you smell more than one note at the same time, which is also called a perfume being “dimensional,” and if that also changes over time, for example you first smell something amber, vanilla and woody, then the amber fades back and the wood comes forward while an aromatic feeling comes in from underneath, that is called layering, and it is a feature of a quality perfume. Then there is nuance, which in a perfume means the notes are present not only with their own specific characteristics but together with other qualities. For example, the smell of rose is classic, but making it feel like a lightly spicy rose is a nuance. The other thing I find important is that a perfume is different and unique. I think this is quite difficult and reflects the real quality of the perfumer, because depth and nuance can be imitated, but creativity cannot. In short, if a perfume has the things I have described, we can say it is “roughly” a quality perfume, I think, but it should not be forgotten that the blend of the perfume, the feelings it gives, and so on are also things that directly determine its quality.