magnum opus
Concise Oxford English Dictionary © 2008 Oxford University Press:
magnum opus /ˌmagnəm ˈəʊpəs, ˈɒpəs/
▶noun (pl. magnum opuses or magna opera /ˌmagnə ˈəʊpərə, ˈɒpərə/) a work of art, music, or literature that is regarded as the most important or best work that an artist, composer, or writer has produced.
– origin C18: from L., ‘great work’.
'magnum opus' also found in these Oxford entries:

