Originally Posted by Mango

Ok, I see now. You are in discovery mode. There is a lot of chamber material outside trio and quartet that's just as good if not better. Apart from Schubert's String Quintet D 956, as you will know there are several Mozart quintets. These are mostly superb. In fact, Mozart quintets are generally reckoned to be better than his quartets. Incidentaly, for the Mozart quintets, the set by the Juilliard String Quartet is excellent.

Schumann and Brahms piano quintets are really wonderful. Of all piano quintets, I think the one by Brahms takes some beating. It's one of my favourite pieces.

There are very many duets involving piano and violin or cello. I mentioned Schubert's D 934.

Also, Schubert's and Mendelssohn's Octets are very good.

Some of Brahms sextets are good.

Dvorak wrote some good chamber music.

BTW, if anyone tells you that Bartok's String Quartets (6 of them) are the best since Beethoven, don't believe them. From what I gather of your chamber music tastes, I reckon you'd be rather disappointed (if not a little surprised) with Bartok's. They are reckoned to be superb but I can't get my mind round them. They do not ahve sufficient melody for my tastes. Shostakovich's string quarters are generally more accessible, and a better start on the 20th Century material than Bartok, but watch out as not all of them are good.