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.

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.

The last pair of Ken-Rad's that I tracked on Ebay went to Mr Blume of Coincident fame. He must like them in his setup, unless they were a gift for someone else. Yes, I am a busy body, so don't start.