Inspired by Best Series , but strictly limited to series that are designed as trilogies.

PLEASE READ THE FOLLOWING

☞ NO GENRE LIMITATION as long as the series is a trilogy.
☞ NOT including series with works set in the same universe yet not exactly related to one another and happened to have 3 books in total.
☞ NOT including series with plans of more installments yet only have 3 books due to the death of its author; e.g.: The Millennium series.
☞ NOT including a piece of work that got split into 3 volumes; e.g.: The Lord of the Rings.
☞ Trilogies with prequels, sequels, companion books, short stories, novellas; as long as the core series is in fact a trilogy, are acceptable for this list.
☞ Each first book of the trilogy (#1) REPRESENTS its entire trilogy.¹
☞ PLEASE make sure you checked the entire list before adding new books to verify what you wanted to add isn't in fact already listed; having duplicates is tiresome.
☞ PLEASE TRY to add the edition with the book cover that's most easily recognized.²
☞ PLEASE TRY to only add books from completed trilogies.³
☞ PLEASE DO NOT add titles that aren't the first book of the series or prequels such as #0.5.¹
☞ PLEASE DO NOT add box sets; as to avoid duplicates.¹
☞ PLEASE DO NOT add works that are comics, graphic novels, manga, manhwa, manhua, or any other works similar to those types.

Friendly reminder: PLEASE, PLEASE JUST THE FIRST BOOK OF THE SERIES.

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¹ This is really just to simplified the list: Not all trilogies have box sets editions, and trying to decide how to rank books within the same trilogies adds unnecessary complication; people need to focus on ranking which trilogies they like the best, and NOT which book of which trilogies they like the best; there is a difference. You may not think highly of the first book of the trilogy you are overall very fond of, but remember, you won't be voting for that book, you are voting for it because it represents the trilogy as a whole.

² The one that'd show up during search result; using rare editions may result in the same title being overlooked and duplicated.

³ Things don't always go according to plan; what was originally planned as trilogies may end up having more books in the end, and therefore no longer being trilogies.