Dominic's answer is right to the point. Tea auctions and competitions
(mainly for Taiwanese oolongs and much less for pu-erhs) can give a
good indication what the informed market regards as the best tea (of a
given type) for that year (or season). Unfortunately, this tea is often
not readily available to the average customer.
That temporary aspect is very important. Tea being a natural product
(and tea masters being humans), each batch will turn out differently.
Just as a tea garden in Darjeeling might produce one batch that takes
the crown for the year, it might not perform very well the following
year. It might also produce rather average tea throughout the "common
grades" (i.e. the stuff regular people can buy through vendors).
And after all, if you're drinking the tea, the judgement lies in your
hands. For example, I had a customer ask me for MY personal favorite
tea and I told him (IMO a very good Darjeeling First Flush). He tried
it and it turned out that he prefers his teas fuller and opted for a
different tea. (I like an ethereal character in a Darjeeling, but it's
not everyone's cup of tea, so to speak)
If you're getting into pu-erh, though, it gets really fuzzy. Suddenly,
it's not only the quality of the original cake, but also storage
conditions, etc. that have to be taken into account. The same tea (i.e.
the same recipe and vintage), just stored differently, can be much
better (or worse) than other cakes of that production.