Which Latin phrase meaning 'great work', refers to the largest and best achievement of an author?

"Opus magnum" is the expression, although it might sound less pedantic to simply use "masterpiece", or perhaps French "chef d'oeuvre" -and the fact of using the adjective after the noun (opus = piece/work ; magnum = big/main) is natural neither in English nor in Latin itself.

Magnum Opus;

Magnum = great, magnificent, amazing

Opus = work, function, operation

opus magnum..........Indeed, it has, in a way or another, the meaning of " A masterpiece", i guess

Yes, I agree with all the other scholars, it is 'magnum opus' or 'opus magnum' (word order doesn't change the meaning).