Chamber Music - your best pick.

Be it the solo, duets, trios, quartets or quintets.
What is/are your favorite(s) in this gener?
What are your recommendations - classical, romantic or 20th century composers.

Most played lately in my system is Shostakovich String Quartets.
Others that get a lot of playing time are Brahms, Dvorak and Schubert.

Would like to check out something new that I haven't heard before. I was looking to pick up some Elgar string quartet and piano quintet next. Any other recommendations? Thanks.