Most desirable vacuum tubes for SET amps

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

I have a Decware that runs the EL34, a pentode tube, in triode.  The sound is unbelievably sweet and wonderful.  With the amp I have the ability to tube roll and tried some KT-88's.  The KT-88 totally changed the sound and made it sound too bright, too sterile and the musicality went away.  Look forward to trying other tubes to see what, if any, sound better.  I also have a Dynaco ST-70, also running the EL34 in ultra linear.  I love the sound of this amp as well but if I listen to the EL34 in SET triode and then listen to the Dynaco, the Dynaco then sounds very mushy.  Some may disagree but to me it sounds like Polk speakers, which to me are pure nasty. 

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 

I also have a Dynaco ST-70, also running the EL34 in ultra linear. I love the sound of this amp as well but if I listen to the EL34 in SET triode and then listen to the Dynaco, the Dynaco then sounds very mushy.

Is the Dynaco sorted out properly? Older ST70s will need all the filter capacitors in the power supply replaced and everything spec'ced out to really assess that.

300b amp:

Psvane Acme 805
NOS Russia Meltz 1578 in the 6sn7 
NOS sylvania vt229 in 6sl7 

As I understand it, the 300b tubes have the smallest impact on my particular amp. They’re next (Psvane Acme) at over $1,000 for a pair. I hope they’ll be worth it.