there's older pieces i'm very fond of (Beethoven's Sixth, the Hungarian Rhapsodies, ktp) but for me it doesn't become interesting enough until Alban Berg. Messiaen & Penderecki are my favorites from later on, although almost any in the mid-to-late 20c appeals to my ear. i especially like those who are trying to combine things (e.g. Messiaen took from birdsong & the East & old church music) to express their vision.
unfortunately, too much of classical music (not unlike rock or jazz) merely exists to propagate an idiom.
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"I have always found that Angels have the vanity
to speak of themselves as the only wise; this they
do with a confident insolence sprouting from systematic
reasoning." --William Blake