What Oldfogey mentioned is exactly my thoughts too. There is no
best sounding tube, but imo, there are worse sounding tubes.
There will be types of tubes that are clearly a cut above the rest.
And those are the types mentioned by OF. I have been using ARC products as phono stage for the past 20 years and the ones I have loved,
and matched my system, and more importantly my bias, are the Telefukens and next, the Mullards. I have tried also the Amperex USN-CEP(fabulously lush sounding tubes- I've never tried pot in my life but I would
imagine that is what it would do to me, these tubes), Siemens 1950s CCa's, (great dynamics and utterly clean sound ), Mullard 2492/6922s (
currently my 2nd fave tube of this type, and my holy grail and mother of
all 6922s, the Telefunken CCa-Gabl ( it basically sums up all the good
traits of the tubes that I just mentioned - clean, dynamic, excellent
soundstage, and a beautiful emotionally charged midrange ).