Beautiful Classical Smaller Pieces, duo,trio,etc

Well Schubert I'm not one for deciding good, better or best of anything, it's more a parlor game than anything BUT Schubert's string quartets are so very special as a genre of music by a single composer that, to me at least, as a body of work they have such a unique special character that can't be decribed or put into words, just an amazing accomplishment for a single human being. His deep sensitivity is keenly projected in his music. He isn't alone but he certainly is quite special in his own unique way. If Brahms say he isn, well I sure wouldn't argue with that!

The other make changes and develop themes.
Schubert comes at you from 20 directions at once and they are all related .

Many great suggestions. Glad to see mention of the Nielsen Quintet and of
Janacek, one of my favorite composers. Janacek's "Mladi"
(Youth) for wind sextet is a gem and highly recommended. Schubert's
"The Shepherd On The Rock" for soprano, clarinet and piano is
one of my favorite chamber works; as is his "Octet" for winds
and strings. Poulenc's "Sextet" for piano and winds is fantastic.
Prokofiev's "Sonata" for flute and piano. In a more modern
vein (beauty is in the eye/ear of the beholder):

Ligeti, "Ten Pieces" for wind quintet.
Elliot Carter, "Sonata" for flute, oboe, cello and harpsichord.
John Adams, "Road Movies" for violin and piano.

Frogman, re Janacek, on hope you've played him or looked at scores.
I keep trying to spread the gospel on Leos but folk find him too "dissonant" .
I hear him as the epitome of that old-time Czech tonality
, as far as my amateur ear can tell he writes at both extremes of the staff and through fabulous technique makes the whole seem right down the middle. To me he is sui generis exemplified , ever fascinating . A true musical genius !

Schubert, I have been privileged to play several of Janacek's works over the years; most recently "Sinfonietta". "Mladi" was a staple of my woodwind quintet's repertory; with the addition of the bass clarinet, of course. Janacek was one of the most individualistic composers in all of music and while his music is, as you point out, dissonant it always remained tonal and rooted in Romanticism and the folk music of his native Chekoslovakia and of Russia. For me, one of the most fascinating aspects of his music is that he is sometimes referred to as the first Minimalist composer with the use of short repetitive or recurring motifs throughout a composition. Truly a musical giant.