To be completely honest, I can't think of any truly great performances on digital. I can think of great performances transfered to cd that sound great:
Hungarian S.Q.- their complete Beethoven is the best studio recorded set I know of. No compromises here. Their Mozart and Haydn is masterful as well. Check out their Brahms quartets, fantastic.
The Griller S.Q. on Dutton Labs, fabulous mozart and Haydn. One of the best examples of British string chamber musicians on record. Real elegance and style.
There is a live recording of the Quartetto Italiano playing a recital of Mozart d minor, Haydn lark, dvorak american, and debussy quartet(?). organic, and the best quartetto italiano recording I know of. Their Phillip's recordings all sound sterile, thin, static and boring to me.
The Budapest S.Q. live at the library of Congress, esp. the late quartets "live" readily avail at any tower records. the budapest at their finest, human, full blooded, deep performances. I think it is on bridge records(?)
Capet S. Q. playing anything.... now this a completely, much higher level of artistry. Spiritual, deep, elegant, stylish, human, organic, realness, true string technique/art, check them out. Their late beethoven is stunning. Also Mozart is pristine. Finest Schumann too.
I would pay Busch S.Q. lip service out of respect and admiration and awe, but find when it comes to pleasure listening (which is the only I have time for) I find it a little tiresome.
Happy listening!