The Cleveland Quartet
"Beethoven String Quartet op. 130 & Grosse Fugue" (Vinyl)

This is part of the older generation (Weilerstein) Cleveland Quartet's Beethoven recordings. For the life of me I still don't know if this quartet recorded Beethoven's early quartets or not.
Their playing is magnificent; I'm certainly partially biased, having studied chamber music with Peter Salaff for four years, but beyond this, the Cleveland Quartet has always been marked by sparkling, intimate and nuanced performances unlike any other quartet.

Beethoven was going all prog rock with his quartets by the time he made it to his opus 100's. The form becomes secondary, movements begin to sprawl and I think it is a very good barometer that when you understand and appreciate Beethoven's late quartets you've reached a modicum of understanding of classical music.