I guess for your own use it would have to be whatever flavor of sound you like.For re-sale thats a little different I guess,maybe NOS,Mullards,Telefunkens,RCA,any pairs,quads of any 12at7-12ax7 ( and many others ) etc etc..or el34 quads of Mullards,Telefunkens seem to bring big $$$.I am sure the list goes on. I've seen Telefunken 12ax7 smooth plates NOS go for some incredible asking prices.Also El34 Mullard quads are in the big $$ league too...
what are the most desirable tube types to collect.
as always seeing different tube types and brands being offered, if you had the opportunity to "cherry pick" a certain type of tube by a certain manufacturer...what would your wish list be??? Of course we're speaking about signal and power tubes...then whatever type floats your boat next...