Most desirable vacuum tubes for SET amps

The el 34 is a good sounding tube but is a pentode and  not a (Innately) triode (Although it can  be re-wired to triode).
It is 'indirectedly' heated as opposed to 'directly' heated triode (DHT).
I believe that the OP was referencing specifically to the niche of DHT tubes.
Charles 

Again no clear cut answer. Most desirable to who? One could certainly say the 300b is the most popular DHT tube. There are numerous current production brands (With various price categories to choose from) 300bs available. A sign of demand if nothing else. There are probably more 300b amplifiers produced than with any other DHT tube. As has been pointed out, the vintage/original Western Electric 300b is highly coveted and extraordinarily expensive. So to a certain niche of W.E. tube seekers  they are very desirable.
Charles

+1 testpilot. Any one of the listed tubes can sound truly excellent if careful attention is given to speaker selection. The higher power transmission output tubes such as 845 and 211 can be magnificent  sounding in the hands of a talented/skilled builder.

Same is true of low watt 45,50,2A3 etc tubes. Ultimate results are fully dependant on the designer/builder and quality of the parts utilized. I just do not believe that any one single tube can be declared the absolute best. There are a multitude of factors to be considered.
Charles