Just select whatever your highest quality music is, or whatever the max is that your gear supports. I really doubt you'll hear a difference, but you could do blind tests if you want to. I don't bother with WASAPI since it stops anything else making sound and it doesn't sound any different to me.
He's looking to get "perfect" sound. The windows mixer can do some harm to things. Either way, I doubt he has appropriate equipment needed to hear the difference anyways.
To the OP. set it 16bit 44.1khz and also tick "Allow applications to take exclusive control of this device". This will allow an application to adjust the output if nessesary. Rarely will you be listening to anything better than CD quality sound. A DVD or Blu-ray will be your typical higher bit and sample rate material.