At last! We reach the bottom of this extremely non-all-inclusive list. Along the way I have, consciously and otherwise, left out significant and even very good composers. One simply has to make an end somewhere. These three guys could all probably be left off this list without anyone noticing or caring, but I needed someone beginning with a Z, and they were about equally obscure...
Zarlino (Gioseffo) was a 16th century composer who wrote highly polyphonic motets and madrigals, all according to a set of strict rules which he set down in several important treatises on music theory. He developed a mathematically-precise system of tuning called just intonation, which was later refined (in the time of J. S. Bach) into the currently-used equal temperament. He also explained why parallel fifths are a no-no (hint: the upper voice disappears in the overtones from the lower voice).
Zelenka (Jan Dismas) was a Czech Baroque composer comparable in some ways to J. S. Bach. He was known for his daring harmonies and expert counterpoint. For the last 10 years of his life he composed church music for the city of Dresden. His surviving works include half a dozen exceptional Masses, four Requiems, oratorios, psalm settings, and instrumental works.
Zemlinsky (Alexander von) was a famous Austrian conductor who spent his last years in exile and obscurity in New York, USA. Best known for his Lyric Symphony on texts by Bengali poet Tagore, he also wrote operas, chamber music, songs, symphonies, choral music, and a ballet.