I was astonished by the number of posts on the 6SN7. As a designer of high-end tube audio gear (coffmanlabs.com) I thought it worth sharing my experience from the design side. YMMV And just to make it clear--I do plan offer some of my products with the 6SN7; I have around 4,000 to 5,000 vintage pre-1970's 6SN7's sitting in boxes in my storage.
The 6SN7 has some of the lowest 2nd harmonic distortion available when run at the right parameters. However, there are very good alternatives. The 6CG7/6FQ7 (and 8CG7) are viturally identical and test out nearly identical to the 6SN7 in every respect from the audio perspective. If the 6SN7 is too expensive, the 12SN7 is a great alternative as well. And don't forget the 6J5 which is 1/2 of a 6SN7.
However, I actually do not like using the 6SN7 as a driver tube for high-voltage hungry SET tiodes (300B, 2A3, 845--etc.) The 5687 has better distortion charactaristics at high drive voltages, and because of the low plate resistance it is perfect for choke loads. Another equivalent circuit, if you must use the 6SN7 is to drive a power triode, is to use a 6SN7 to drive a 12B4 and let the 12B4 do the heavy lifting. (The 12B4 is one of the all time under-rated tubes IMHO.)
Where I do like the 6SN7 is as a driver tube for typical pentodes like the EL84, EL34, 6L6, 6V6 and the like. As an input buffer tube it is great. I wouldn't use it for the basic drive tube for a pre-amp because an additional cathode follower is needed to get the low output impedance in a typical circuit. I'm not a fan of cathode follower circuits, or any circuit that uses feedback. No matter how good they are I can hear the change to the phase in the harmonics--left over habit from my classical violin days.
My favorite 6SN7: That's easy--the TungSol round plates. Unfortunately, I sold my last pair to Korea for $1000+ when I needed some new manufacturing equipment during the Coffman Labs startup phase. So my second favorite--and the one I turn to--is the Raytheon Black Plate 1960's vintage VT-231 Mil Spec. Most neutral and realistic, again IMHO.
Again, all these opinions are heavily colored by my experience, listening preferences, and the rest of my system. I don't expect them to necessarily fit with everyone else, but you may find them interesting.