Although others may consider as Ludwig van Betthoven's (1770-1827) greatest work his Eroica Symphony, which introduced the Romantic Period, or his Fifth Symphony with its unforgettable four-note theme, or his Ninth "Choral" Symphony with its memorable "Ode An die Freude," Beethoven himself regarded as his greatest work his Missa Solemnis in D. In the opinion of many, it is second only to Johann Sebastian Bach's High Mass in B-minor.
Curiously, Beethoven admired most highly as the model of true Church music the Renaissance composer, Giovanni Pierluigi da Palestrina (ca. 1525-1594), regarded as the most Catholic of composers. When the Council of Trent (1545-1563) was about to ban polyphonic compositions at Mass in favor of only the Church's own Gregorian Chant, Palestrina composed the Missa Papae Marcelli, whose polyphony was of such beauty that the Sacred Council withdrew consideration of the ban.
http://www.traditio.com/comment/com1912.htm#191227