the "best" is a very subjective term, people have varying tastes and therefore, differ in their opinions on the best of anything.
I've found that there are two types of brands or vendors. Ones that are specialized in a certain type of tea, and others who cover all areas of tea.
Adagio is of the second type. They carry multiple kinds of tea and have a good selection of everyday teas as well as some more uncommon teas.
Other vendors that cater to a single type of tea, such as Den's Tea for Green Teas, specialize in only that type of tea. They can then offer a wider variety of that one type of tea.
It would seem to me that if you want to buy the best tea of a certain kind, then you would be better off looking and buying thru a supplier that specializes in that type of tea. If you were looking for the best kind of Earl Grey you would not, and could not, find it at Den's Tea.
I think that your question would be better phrased if you asked which was the most preferred kind of tea and which supplier we usually buy this kind of tea from.
For everyday i prefer Moroccan Mint tea. It is a blend of Gunpowder Green Tea and mint. While it is a staple item at many places, i really enjoy Argo Tea's blend the most.
I really doubt that there is, or ever will be, a single brand that supplies the best of every kind of tea.