This is easy - the very best you can get is the Stahltek Vekian, or Opus, or Opus Prime DACs. Unfortunately, all are megabuck. But you can rarely pick up a used Stahltek for something in the vicinity of 10K. The Aria is more affordable and give most of the performance, but have never seen one on the used market.
Best DAC for CD redbook playback from a PS Audio Perfectwave transport
I just purchased a PS Audio PWT and will use it to listen to redbook CDs. I've read that the best playback is with an I2S hdmi cable. So, a suitable dac would be one with an I2S input, such as either a Wyred4Sound dac or a PS Audio PWD. But, if it's just for redbook CD playback, can I suffice with an older MK1 version or do the updated versions (which I presume are primarily geared toward improving computer based systems) also significantly improve CD SQ? Again, it's just for CD playback.