Wolfgang Amadeus Mozart: Divertimento K. 563; Gideon Kremer (violin), Kim Kashkashian (viola) and Yo-Yo Ma (cello) (CBS recording) I cannot recommend this recording too highly, it is so superlative.
Johann Sebastian Bach: "Double" Concerto for two violins in D minor ("Great Performances" series) with Isaac Stern and Yitzhack Perlman on violin. Dueling Strads!
Franz Schubert : Sonata in g major,k op. 78, D 894 and sonata in B flat major,k D960: Grigory Sokolov, piano (Opus 111 recording)
Franz Schubert: Impromptus D899, D935; Lambert Orkis, fortepiano (Virgin Classics, digital). Contemplative without being depressing.
Johann Sebastian Bach: Goldberg Variations; Rosalyn Tureck, piano (Deutsche Grammophon) Rosalyn Tureck is the greatest Bach keyboardist playing today, bar none!
Johann Sebastian Bach: the Art of Fugue BWV 1080; the Emerson String Quartet (DG). Bach wrote this for the organ, but this transcription for string quartet highlights the intellectual complexity, leaves the majesty intact, while adding warmth.
Johann Sebastian Bach: Cello Suites; Pierre Fournier (DG) It's a wonderful recording, but if you can find a Yo-Yo Ma recording or Janos Starker's recording, they are also terrific.
Hector Berlioz: Requiem and Te Deum; London Symphony Orchestra, Sir Colin Davis conducting (Philips) give your speakers or headphones a real workout! This work goes from pianissimo to fortissimo, and is always breath-takingly beautiful.
Hector Berlioz Symphonie Fantastique; Concertgebouw Ochestra, Sir Colin Davis (Philips) This symphony still sounds new.
Beethoven's trios for piano, violin and Cello.
Beethoven Piano Concertos: Look for recordings by Murray Perahia. The Wilhelm Kempf recordings have been remastered and released, but the sound quality is a little lacking in the recordings I have heard.
Beethoven Sonatas: Look for recordings by Peter Serkin, just remastered and re-released.
Frederic Chopin: 4 Balladen; Krystian Zimerman, piano
For quiet concentration, try to find a remaster of Julian Bream playing baroque music on guitar.