All just my personal opinion
Beethoven: String Quartet op.95 — comparing recordings with the following ensembles: Amadeus Quartet (1960), Artemis Quartet (2005), Busch Quartet (1932), Chiaroscuro Quartet (2012), Emerson String Quartet (1997), Endellion String Quartet (2008), Guarneri String Quartet (1968), Melos Quartett (1984), Quartetto Italiano (1971), Takács Quartet (2004)
Beethoven: String Quartet op.74 — comparing recordings with the following ensembles: Amadeus Quartet (1960), Artemis Quartet (2011), Emerson String Quartet (1997), Endellion String Quartet (2007), Guarneri String Quartet (1967), Melos Quartett (1984), Quartetto Italiano (1971)
Lieder recordings (Beethoven & Schumann) with Fischer-Dieskau/Moore, Wunderlich/Giesen, Goerne/Schneider, Terfel/Martineau, Bostridge/Drake; Schumann: Piano Quintet op.44, Piano Quartet op.47 — Melnikov/Jerusalem Quartet, Levine/LaSalle Quartet; Webern: 6 Bagatellen, 5 Movements for string quartet, op.5 — Hagen Quartett
Beethoven: String Quartet op.59/3 — comparing recordings with the following ensembles: Amadeus Quartet (1959), Artemis Quartet (1998), Emerson String Quartet (1997), Endellion String Quartet (2007), Guarneri String Quartet (1966), Guarneri String Quartet (1992), Kuijken Quartet (2006), Melos Quartett (1984), Quartetto Italiano (1974)
Beethoven: String Quartet op.59/2 — comparing recordings with the following ensembles: Amadeus Quartet (1959), Artemis Quartet (2008), Emerson String Quartet (1997), Endellion String Quartet (2007), Guarneri String Quartet (1966), Guarneri String Quartet (1991), Hagen Quartett (2010), Kuijken Quartet (2009), Melos Quartett (1984), Quartetto Italiano (1974)
Beethoven: String Quartet op.59/1 — comparing recordings with the following ensembles: Amadeus Quartet (1959), Artemis Quartet (2005), Emerson String Quartet (1997), Endellion String Quartet (2008), Guarneri String Quartet (1966), Guarneri String Quartet (1991), Hagen Quartett (2003), Kuijken Quartet (2009), Melos Quartett (1984), Quartetto Italiano (1974)