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...
Telefunken ECC802S if you can find them, IMHO. They won't come cheap though.
Gtthrush. Great choice... The siemens Halske 5814 is about as good as it gets in the 12au7 family.