Budapest Quartet
Beethoven String Quartets opp. 18, 59, 95, etc. (CD)

It's very revealing to listen to these old quartet recordings in comparing them in where string chamber music is at today. I would argue that the Budapest Quartet was one of the first world-famous string quartets of the modern era, certainly in part because they existed at the dawn of modern recording techniques. From their loins sprang the Juilliard Quartet, the Guarneri and from there it's been a short genealogy to our present day.

These CDs are of the Budapest's 1940's mono recordings which in itself is very interesting to hear.
Playing styles have changed in subsequent generations, but these guys set the benchmark for ensemble and cohesion.