As others have said, it depends on the repertoire!

For Beethoven I prefer the Alban Berg's recent live recordings that were issued on EMI. Their performance of Op. 131 in C# minor is awesome. Same goes for the great A minor quartet too. I prefer them overall to the Takacs. For Mozart's 'Prussian' quartets I love the Alban Berg's late 1970s recordings on Teldec and for the 'Haydn' quartets it's got to be between the Chilingirians and the Mosaiques on period instruments. With Schubert it has to be the Lindsays (although I prefer the Amadeus Quartet for the great string quintet in C).