here's a good introduction I always copy+paste:
Try Beethoven's 3rd and 7th and then 9th symphonies. Mozart 39, 40, 41. Tchaikovsky 4 and 6. Dvorak 8 and 9. Schumann's and Brahms' symphonies, Haydn's Paris Symphonies, Bruckner's 5th and 7th and 8th, Mendelssohn's 3rd, 4th, and 5th.
Tchaikovsky Piano Concerto no. 1. Beethoven piano concerto 4 and 5. Mozart piano concertos 19 through 27. Bach's Keyboard Concertos (I prefer the versions with piano, look up the ones performed by Schiff). Schumann's Piano Concerto. Rachmaninoff's other piano concertos (1, 3, 4).
Beethoven and Brahms' and Tchaikovsky's and Dvorak's and Mendelssohn's violin concerto. Bach's violin concertos and double concerto.
Beethoven's violin sonata 7, 8, 9, and 10. Bach's violin sonatas and partitas (1, 2, and 3 for both). Mozart's violin sonatas. Brahms' violin sonatas.
Dvorak's cello concerto. Schumann's. Haydns'. Beethoven's cello sonata 3 and 4. Brahms' cello sonatas. Bach's cello suites. Prokofiev cello sonata. Mendelssohn cello sonata 2.
Beethoven's piano sonatas, all of the ones that have a named title (eg Pathetique, Waldstein, Moonlight, Les Adiuex, Tempest). some Mozart piano sonatas. Bach's Well Tempered Clavier, both books. Schubert's piano sonatas D.960 and 959 and 958(?). Prokofiev piano sonata 6. Chopin Ballades and Etudes 10 and 25.
Beethoven's string quartets 12-16. Mozart's 'Haydn' string quartets and string quintets. Brahms' string quintets. Dvorak's string quartet 12. Mendelssohn string quartet 6.
Bach's cantatas, 51 and 140.
Try a couple from each and keep exploring whichever form you like the most at that moment. Feel free to come back and ask whenever you can't decide and/or need help deciding on recordings (the recording, as in the interpretation and performance, matters a ton, as it can change the sound, power, and emotions of the music dramatically). Come back when you've listened to it all. Enjoy!