Maurice Ravel (1875-1937)
It's less Ravel's harmony (how he combines notes) that interests me, more his command of timbre (sound colour) through Russian-influenced orchestration, and his fathomless facility with large-scale musical form.
Listen to the opening of Rhapsodie Espagnole (1907-8) and how the ostinato (repeated musical fragment) changes colour continuously, before your ears. Wonderful.
Ravel's La Valse (1920) subjects a waltz dance to transformation by the distortions of dream. The result is a mesmerising, multi-layered gestalt.
According to one of Ravel's few pupils: Ralph Vaughan Williams, Ravel's motto was Complexe mais pas compliqué (complex but not complicated). That speaks volumes.