It is not so simple a question as you might at first blush think, although some decades ago, the answer may have come to mind a bit quicker.
So what IS your favorite sci-fi or fantasy trilogy?
But those are not quite valid answers any longer.
The FOUNDATION trilogy was later expanded to include no fewer than seven books, while that EARTHSEA three-parter currently stands at five books, all told. Even Tolkien‘s MIDDLE EARTH trilogy was intended by its author to be read as a single volume, only to be split into three segments by publishers in later years.
So as I say, the question is not one to be answered hastily. But I DO urge you to respond as quickly as possible, before your favorite three-parter is suddenly turned into a four-parter by its author!
As always, we’ll choose one random commenter to choose a book from our stacks.