Symphonies are great and all, but incredible music can be made with just a few musicians playing. Chamber music is music with a very small musical group, usually one individual per instrumental part. Creating a lot out of very little. This is a far from definitive list of chamber classical pieces that I think are great.
Sorry in advance that there are so many string quartets, I just listen to those a lot.
Probably one of my favorite classical pieces to listen to in general, Shostakovich's 2nd piano trio is one of his lesser known pieces but it contains the same incredible sense of energy and emotion that one may expect with his chamber work. Dedicated to his best friend Ivan Sollertinsky, who died as he was evacuated during the Siege of Leningrad, the work features a strong sense of grief and overwhelming emotion. The piece bounces between frenzied, dance-like melodies, and drawn out dissonant sections, and packed with klezmer elements. (The video embedded only includes the fourth movement, my favorite, but if you want to listen to them in order then all movements by the Greenwich trio are uploaded seperately on YouTube. Your listening experience is your own!)
It's Shostakovich's most famous quartet, but being famous doesn't make it bad. It's an incredibly emotional work, written in a time when he was under emotional turmoil, though while emotional turmoil is a frequent friend when listening to Shostakovich, his fear while writing this was particularly acute. It surges back and forth from incredible fear, almost violent, to a sense of all-encompassing bleakness. It also features an almost obsessive use of the DSCH motif (D, E flat, C, B), a musical cryptogram representing Shostakovich's own name. It's not an easy listen, but one I always encourage.
(If you listened to his 2nd piano trio, the 2nd movement of this quartet also features a melody taken from that piece! In fact, snippets of melodies from other works by Shostakovich can be found in this quartet almost constantly. Take that how you will.)
This work is incredibly joyful to listen to. While each movement has its own emotional portrait painted across the course of it, the one thing this whole work has in common is the easiness and happiness that every instrumental part seems to emanate. Written while Dvořák was spending some time abroad in North America, the quartet contains undertones of the emotional freedom and wistfulness that comes with new experiences, along with a sense of the adventurous and very catchy melodies. An easy listen that's very approachable for the average listener.
Toss out every preconception you have about classical music before listening to this or else this piece will shock you. (Though... this piece will probably still shock you.) Black Angels is one of the more well known contemporary pieces of the incredibly odd type, and though it expresses itself very differently than a lot of classical music you may think of, it still is filled with raw emotion and meaning. It is often interpreted as a threnody towards the horrors of the Vietnam War. It's not for everybody, but it's worth a listen.
One of Schubert's more famous works even outside of the classical fan realm, this quintet is almost leisurely and playful in the way the musics play off of each other. It's named the "trout quintet" because some melodies, mainly in the fourth movement, involve variations on a lied, previously written by Schubert, called "the Trout." Delightful, approachable, and you're bound to have fun while listening.
An absolutely mesmerising piece, this string quartet is the only string quartet that Ravel ever made. While modeled after the structure of Claude Debussy's own string quartet, Ravel created his own unique work with the ideas and developments within the music itself. Funnily enough, while Ravel dedicated it to Gabriel Fauré, his teacher, Fauré ended up disliking it and Debussy ended up liking it. Ravel tries to adhere more towards traditional forms and instead experimenting within those boundaries, creating something great and unique. I admire every piece of Ravel's for the unique texture that his works alone carry, and this piece is no exception to that. I especially love the second movement, with the boatloads of vibrant pizzicato (plucked notes) and light and airy melodies.
A dark and dramatic work, Schubert's Death and the Maiden quartet is jam packed with interesting moments, with high contrasts and memorable melodies that make the piece almost lyrical. The piece jumps from fortissimo (very loud) to pianissimo (very soft) quite often, adding to the dramatic effect. The sounds are rich, with certain movements being dance-like in their rhythms. The sense of foreboding in this piece can be attributed to the fact that Schubert wrote this as he realized he was dying from illness. Indeed, the last movement alludes to Schubert's lied "Erlkönig," in which a young boy pleads to his father for help from the Erlkönig, but dies in his father's arms by the end of the increasingly frantic song.
Can you tell who my favorite composer is yet? Joking aside, this piece was the very last thing that Shostakovich ever wrote, doing so on basically his deathbed. Like a lot of Shostakovich's late work, the piece is rather minimal in its contents and developments, is slow, crawling almost, and carries an overtone of eerieness and tired despair. The viola as an instrument doesn't have a lot of repertoire compared to, say, the violin, which is a shame to me as a viola player. So it's nice to see that Shostakovich created such an incredible work for it.