When I was in high school my older brother made a list of his top 10 to start me on my journey of discovery. Here are some, with my own additions;
Bach…The Musical Offering, Mass in B minor
Mozart…Symphony #40 in G minor, Clarinet Quintet in A.
Haydn…any Salomon London Symphonies
Beethoven…The odd numbered symphonies from 3 to 9, late String Quartets, late Piano Sonatas (but they’re all good).
Schubert…String Quintet in C…called the Great for a reason! Also late piano pieces.
Chopin….Sonatas, Preludes, Ballades, etc.
Leaping over the mid 19th C. to Brahms…Piano Concertos 1 & 2. But Mendelssohn Symphony 4 is great, and Wagner has a few fans, I hear.
Mahler…Symphonies 1, 2, 6, 9. This is heavy stuff!
Ravel….Afternoon of the Faun
Stravinsky…The Rite of Spring
Bartok…Concerto for Orchestra …
Then there’s Shostakovich!
and….IMO, why did you wait so long!