Tomás Luis de Victoria
Spanish composer (c. 1548-1611), in the reference literature arguably the most famous Spanish composer of all time and the peer of Palestrina and Lasso in late Renaissance polyphony. Trained in Rome at the Collegium Germanicum, chaplain to the Dowager Empress María in Madrid, he closed his work with the Officium Defunctorum of 1605, regarded as his masterpiece.
Why this wins its question: Renaissance polyphony is told through Palestrina; Victoria — his Spanish peer by the reference literature's own ranking — is the under-told member of the triad, stated here with the assessment attributed rather than asserted.
Claims
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Victoria was born in Ávila around 1548, trained as a cathedral choirboy, and entered the Jesuit Collegium Germanicum in Rome in 1565, where his path crossed Palestrina's — whom he possibly studied under and whose post he later succeeded to.
Returning to Spain in 1587, he served as chaplain to the Dowager Empress María at a Madrid convent, and on her death composed the Officium Defunctorum (published 1605), regarded in the reference literature as his masterpiece; he died in Madrid in 1611.
The reference literature ranks Victoria with Palestrina and Orlando di Lasso among late Renaissance composers and calls him arguably the most famous Spanish composer of all time — a judgment quoted here as the sources' assessment.