What's the best (or is there a "best") DAC input to use?

AES/EBU, a professional interface that was likely used for the master your music is derived from, will be best

USB is problematic in practice - besides the noise issues noted above, there can be ground loop problems

Toslink avoids the above, but...

then there is the issue of Redbook vs. Hi-Res

Schiit has a new product Eiter(?) and there is the Iso Regen, etc. and no doubt other manfs. have things (besides hyper-$$ cables) for USB

I would investigate the 2 boxes as both companies have return privileges

BTW, Firewire was good

the future is not always better

AES is best in my particular setup.

I have tried almost every connection for a DAC.  Of course, it depends on what the DAC will accept, but over the course of at least 100 DACs, this is my take on the best:

1.  I2S
2.  Firewire
3.  AES/EBU
4.  Coax with BNC
5.  Coax with RCA
6.  USB
7.  ATT Glass Fiber
8. Toslink

Very enlightening responses by all. Sincerely appreciated. Besides the fact that the DAC itself often wouldn't have every conceivable input, the transport also wouldn't have every conceivable output. Amazing how this hobby's possibilities and the quest for audio nirvana is seemingly never ending, because given the responses thus far, finding the right combination of DAC, transport and cabling between the two can be rather challenging.