The Best , make that the all time best, 6SN7.
In a Hollywood twist ending, it turns out that the best 6SN7 is not a 6SN7 at all, it's a pair of 6J5 / 6J5-G / 6J5-GT, each of which, electronically identical to one half of a 6SN7. You can buy ready made adapters on EBay,
which convert two 6J5's into a single 6SN7. 6J5 + 6J5 = 6SN7. I believe the 6SN7 was created for economic reasons, what could be more economical, then a tube, which can take the place of two tubes?
I also believe that it was a step backwards for high-end audio, because the single triode, per bottle, sounds far better than any 6SN7. You can also buy metal based 6J5-GT, which sound way better than Sylvania's metal base 6SN7's and you can usually find a quad of them, cheaper than you can buy the Sylvania 6SN7 W, metal base
and / or the Sylvania 6SN7 A metal base.
I have two quads which beat by a fairly wide margin, the world's most expensive 6SN7, which currently are,
the Osram / Marconi / B 65, metal base 6SN7. I bought a single because I had to hear it. I then used it to recalibrate 3 tube testers (the single tested dead nuts on an Avo 160 tube tester) then sold it.
I saw a pair of the Osram / Marconi's go for $4,000 on EBay, there are currently to pairs offered at the relatively bargain price of only $3,600 for a pair of 6SN7's.
Happy tube hunting!