What is your favorite tube for a preamplifier ?

The NOS RCA 12au7 made a gigantic difference in my CJ PV10 and PV12. It is also less expensive than the other NOS tubes.

I used very expensive mullard NOS 6922's in my Audible Illusions M3A and got only a small improvement, but that could be because the M3A is so good that it is hard to improve.

Well, one of my favourite preamps (Atma-sphere) uses 6sn7s, but my other favourite tube preamp (Joule) uses 6350s which I never heard of - I think they were designed for TVs, not sure, but it sure does make for a nice sound preamp. My other tube preamps have used 6922s and/or 12AX7s. The old ARC SP6 used all 12ax7s I think, and for the time it was a might fine sounding preamp.

I think it depends on the build quality of the preamp more than the tubes. I have had a preamp that used only the 6SN7 family of tubes vs the other that used the 6922 family of tubes. The second pre with the 6922's sounded better because it was a better preamp. I never had a preamp, don't know if one is even available, that would accept both 6SN7 or 6922's which would be a better way to compare them, at least for that particular preamp. I see many posts from people that hate the 6H30 super tubes and others swear by them. It all boils down to the sound that is most pleasing to your ears!