General Terms:
* Magnum Opus: Latin for "great work," often used for a writer's most important or significant work.
* Masterpiece: A work of exceptional artistry and skill, considered the author's best.
* Crowned Jewel: Used metaphorically to describe a work that stands out as the most precious or valuable.
* Signature Work: The work that defines the author's style and is most closely associated with them.
More Specific Terms:
* Bestseller: The book that has sold the most copies, though not necessarily the author's best work from a critical perspective.
* Critical Acclaim: A work that receives widespread praise from literary critics.
* Most Celebrated Work: The work that has garnered the most awards, recognition, or adaptation.
* Most Personal Work: The work that is most deeply personal and revealing of the author's own life experiences.
* Most Influential Work: The work that has had the greatest impact on other writers, readers, or culture.
Ultimately, what constitutes an author's "best work" is subjective and open to interpretation. It depends on the criteria being used: critical acclaim, commercial success, personal significance, or impact on the literary world.