The recordings I've listened most to are Arcanto Quartet/Avenhaus (modern instruments), La Gaia Scienza (period instruments), and Leipzig Quartet/Staier (modern instruments). I don't actually like the piece very much, so I'm not sure how I accumulated so many recordings (10 or 11 at the moment), nor am I sure entirely how best to evaluate them. That said the Arcanto Quartet are a default recommendation in anything they've recorded, which is criminally little; under normal circumstances you'd think record companies would be lining up to get a band consisting of Antje Weithaas, Daniel Sepec, Tabea Zimmermann and Jean-Guihen Queyras to set down the entire standard rep, but apparently not.
Of recordings I haven't listened to, Takács Quartet/Hough and Pražák Quartet/Klánský seems like the strongest contenders offhand.
There is a recording of the string quintet version by the Divertimenti Ensemble (and possibly others), but this is a modern reconstruction, not Brahms's original, which is lost. I thought I had a recording of the Sonata for Two Pianos (usually labelled Op. 34b or 34bis) but apparently not, or at least not one that I ever ripped to my hard drive.