What is the best 6SN7 driver tube?

The best one is the one that sounds the best in your amp (your circuit) and your system, to you. There is no best tube, in general, considering these criterium, as each tube will perform differently under these different conditions and also according to your own personal taste. Some people dislike Sylvanias, some 5692's in general and some RCA's (including the red line and base models). The only "Hall of Famer" that I am aware of is the Ken-Rad Vt-231/6SN7GT, but as to whether you would like the sound of it in your system, I don't know. Try running a search on 6SN7, 5692 and VT-231 at the Audioasylum.com tube forum and you will see what I mean. It will be fun reading nonetheless.

Edie, where is this 6SN7 going, and what function does it perform in your system? Also would help if you explain your reason for wanting to replace the current tube in that circuit ( microphonics, distortion, short life, etc.)

I am not an NOS tube aficianado. But I will say that in the output driver section of my c-j MV-55 power amp, I have tried various old American 6SN7's taken from my guitar amps,plus some originally store-branded NOS ones (probably of RCA manufacture) that I ordered from ATSI. Compared to the new Sovteks at $6 a pop (ATSI price), the differences were too small to worry over in my system (playing through Thiel CS2 2's). Much less to choose from here than in output or input tube selection (where I actually tend to disfavor Sovteks). I can only conclude that either the new tube is competent, or that in the driver position of this amp, at least, the choice is not critical. Whatever the case, they've been reliable, and are certainly cheap enough that it's painless to keep 'em in "to-spec" condition through regular replacement. But as you can tell, I'm one to avoid NOS anxiety wherever I can away with it - I find that doing so helps me to relax and just enjoy my music

Edle: Sorry, I just went to Audioasylum.com and the search feature will be down until Monday or so (they are re-vamping the site). When it comes up again, try searching under your specific amp and the type of tube and you should find many other user/owner comments on various combos (granted that you do not own an oddball one such as I do). Many of the posters will also have their systems posted in a separate area of the site, which will help you to understand where they are coming from. This is second best to trying the tubes yourself, but is a usefull aid.