Best 12AU7?

Can't tell which one is best, but can tell you that the RCA clear tops are no slouch, and are typically reasonable. How any tube sounds will vary a bit with each system and circuit.  

We can generalize about the sound of tubes, but you won't know for sure until you try them in your system.

The RCA Clear Tops are supposed to be good in the treble.  If you want to spend some crazy money, search for Telefunken G 73-R, a very specially selected 12AU7.  I've never heard them, probably never will.

I like Amperex 7316 but don't think they're airy, more of a midrange tube.

Call Brent Jessee and talk it over.  He'll have something for you.

I used the Brimar 13D5 several years ago and loved them.  In my SET, they were very clean and clear without being bright.  I had the RCA clear tops prior and the Brimars were in another league in my opinion with quite noticeably more detail.  Some have said they are a bit warm.  I did not hear that at all but I did not hear any leanness either.  Images were also more distinct.  They are just a great tube, if your sonic preferences reflect the qualities mentioned.  

I have a pair of Telefunken 802S Platinum Matched Pair Best 12AU7 for sale. Please reach out to me.

Telefunkin ,Siemens , all very good nos tubes from Germany