There is much more to say about tubes than would fit into a thread. As was mentioned before, some tubes of days long gone by do have a unique mixture of materials in a pureness you won't find anymore today.
But in Audio it all comes down to the individual judgement of the respective listener/owner about what set of tubes he/she like BEST in a specific piece of equipment under certain circumstances ( circuit, matching with other components etc.).
There is no rule - there is no universal guideline. I for one do look for very specific mechanical criteria in tubes (and tube-types..) I use in amplifiers, because I know about certain direct links between sonic performance in a given circuit and specific mechanical/architectural circumstances in a tube. And this too is depending whether I use them as plate-follower, in SRPP mode, input-, driver- and/or phonostage-tube.
In the very end it always come down to what YOU do like best.
That is all that counts.
There is no BEST tube (type or version) out there.
There is a selection of great tubes for each specific purpose.
NOS or NNS - you will find good and great tubes in both camps (but you are likely to find a wider selection in the NOS-camp .... ;-) ..... ).
Try and enjoy.
