The highest quality conversion possible?

Weiss, Lavry gold or Prism.

You will be looking at about $6000 - $12,000 for a stereo A/D converter. If you have a properly designed and built room with correctly positioned acoustic treatment and very high end monitors, you may just benefit from the 3 or 4% increase in quality over stuff that is thousands of dollars cheaper.

Can I ask why you need "the highest quality conversion possible"? Perhaps I am bias, but I find conversion one of the most overrated, and overhyped pieces of gear in any signal chain. Particularly if you are just starting to build up a collection of gear.

A $5000 microphone will sound at the top of its game in 30 year time...
A $5000 set of converters will be an absolute boat anchor...

Some food for thought.