Just read through this entire thread this morning. Much of this thread has been quite interesting to me.
My main classical focus is on String Quartets (and some String Quintets) playing the music of Haydn, Mozart, Beethoven, Mozart, Schubert, Brahms. Mendelssohn, Dvorak and others from the classical and romantic periods.
While I have been enjoying a number of recordings by the Juilliard Quartet, there are 5 or 6 other string quartets who I tend to prefer.