Bartok's are good, Shostakovitch's are good. By far my favorite string quartet of all time is Schubert's Death and The Maiden quartet! That and the Grosses Fugue of Beethoven. My absolute 2 favorites!
I can't help it, but the late Beethoven Quartet's are on another level (especially op. 131) to me. I haven't heard any other quartet, maybe even composition, that communicates so directly and so profoundly.