Bearing in mind your expressed preferences I think you'd find each of these worthwhile:
Violin: For solo violin Bach's Sonatas and Partitas by Julia Fischer on Pentatone(PTC 5186 072); for violin concerti Tchaikovsky's concerto etc also by Julia Fischer on Pentatone (PTC 5186 095), Vivaldi's "Four Seasons" by Janine Jansen on Decca (475 6188), and the Beethoven and Mendelssohn concerti by Victoria Mullova on Phillips (470 629-2).
Piano: For solo piano Burkard Schiessmann's Chopin recital on Bayer (BR 100 348), Yevgeny Sudbin's Rachmaninov recital on BIS (BIS-SACD-1519), and Artur Pizarro's Ravel recital on Linn (CKD 290); for piano concerti the two Liszt concerti by Arnaldo Cohen on BIS (BIS-SACD-1530), Mozart's 9th and 25th by Alfred Brendel on Phillips (470 616-2), and the firsts of Tchaikovsky and Medtner also on BIS (BIS-SACD-1588).
Flute: A group of 20th century pieces for flute and orchstra coupled under the title "Bridge Across the Pyrenees" by Sharon Bezaly on BIS (BIS-SACD-1559), and the Mozart Concerti by Jacques Zoon on Telarc (SACD-60624) or Sharon Bezaly on BIS (BIS-SACD-153901) Note: the Zoon is just the two complete concerti coupled with the Symphony #41 all on "period instruments" while the Bezaly includes two additional Mozart movements for flute and orchestra, the instruments employed are "modern", and the cadenzas are challenging, or wierd depending on your point of view, newly composed by Kalevi Aho.
Orchestral: Three "mountain" symphonies by Alan Hovhaness on Telarc (SACD-60604), the Vaughn Williams Symphony #5 etc on Telarc (SACD-60676), the Symphony #3 and Cello Concerto of Peteris Vasks on Ondine (ODE 1086-5), Grieg's Peer Gynt suites etc on BIS (BIS-SACD-1591), and another BIS miscellany called "Seascapes" (BIS-SACD-1447) with includes Debussy's "La Mer" and what I personally believe is an even better piece, Frank Bridge's "The Sea".
As you've noticed several of these sugestions also appear on other replies, which just indicates that there's probably a consensus about the value of the music itself and the excellence of those recordings. Some others are a little off the beaten track because for me, and I suspect many collectors feel the same, the great joy of collecting classical recordings is in the discovery of something new and unexpected that speaks to my mind and soul. I envy you the opportunity to hear it all again for the first time.