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

Answer: The right answer is magnum opus, in singular, and magna opera, in plural.

Explanation: Just to elaborate a little on the answer, it can be added that magnum opus literally means "great work," and it is an expression used to describe a literary piece, a musical composition, a painting, a film, or other example of artistic creativity and genius that stands as the best and most important achievement of its creator.