Most neutral, high-resolution 12AU7?

So far, I have my eye on the Siemens - Halske 5814A / 12AU7.

Any other suggestions?

This will be used as the input tube in my Ray Samuels Audio "Raptor" headphone amplifier.
I am not an expert on this tube ,but I have seen mentioned the clear top Amperex 12au7 a few times as being one of the best around....
I think it depends on the rest of the circuit. A tube that might be overly bright or even brittle in one circuit can bring another to life.

Which direction do you need to go with it? I have really liked several. RCA cleartop side getter is one of my faves. Amperex bugle-boy is also nice, but it does tend to brighten up a little more I think. For a new tube I have quite liked the JJ. It all depends...
Gtthrush. Great choice... The siemens Halske 5814 is about as good as it gets in the 12au7 family.