depends what floats your boat – I guess everyone starts out on a different tack – often from film or advert usage..
Bach for the complexity (although there’s not a vast variety, it’s more the combinations that make it great)
Worth looking at distinctive stuff – Janacek Sinfonietta, Bartok Concerto for Orchestra; Mozart Clarinet Concerto. Stravinsky too, and Shostakovich – cello & piano concertos, symphonies are pretty grand – also his jazz suites are well written.
Vaughan Williams is different – Pastoral Symphony. Must say it does take a while to acclimatise to Wagner 🙂
Songs – Schubert, Schumann.
others – where to start? Mahler, Beethoven (4 & 7 are particular favourites), Brahms concertos, Steve Reich possibly? Maxwell Davies; Sibelius Symphony 5; Grieg Holberg Suite; Peter Warlock.