Hmmm, are we talking "series" as in "including trilogies and quartets" here, or does it have to be more than that number? Also, what about works that were intended as one (very long) book but are traditionally broken up into several parts that are published separately (like Tolkien's Lord of the Rings) and books originally published in several self-contained parts but now frequently combined into one omnibus volume (like Stephen King's Green Mile)?
Anyway, starting with the beasts that nobody can legitimately dispute are series and moving on from there, based on the assumption that it's "yes" to all of the above:
MULTI-BOOK SERIES ( >5 INDIVIDUAL ENTRIES)
Terry Pratchett: Discworld
J.K. Rowling: Harry Potter
C.S. Lewis: The Chronicles of Narnia
Sheri S. Tepper: The True Game (all nine books, including the Mavin Manyshaped trilogy and the Jinian / End of the Game trilogy)
TRILOGIES / QUARTETS / MULTI-PART OMNIBUS VOLUMES
J.R.R. Tolkien: The Hobbit and The Lord of the Rings
T.H. White: The Once and Future King
Tad Williams: Memory, Sorrow and Thorn
Mary Stewart: Merlin Trilogy
Stephen King: The Green Mile
JUMPED THE SHARK
Anne Rice: The Vampire Chronicles
Unsurprisingly, almost all of my favorite supernaturally-tinged series are fantasy -- and I read both Green Mile and the Vampire Chronicles for pretty much everything but their horror contents. That said, Rice jumped the shark for me when she insisted on using Lestat (of all characters) as a vehicle for exploring her rapidly altering expressions of faith ... shortly before going BBA and thus earning herself a place on my no-go list once and for all. I still like the first books in the series, though, especially the first two.
In general, it would have to LOTR. Because I've read one copy into the ground.
But this year, my top three Halloween fitting titles would be:
The Women of the Otherworld series. - I believe this was Armstrong's first series. At times it shows, but you can see her developing as a writer (and she works with her editor). What I really love about this series is not only does feature women, but each woman is strong in her own way. And I say this as someone who didn't like all the women in the series. Additionally, the women are friends with each other.
Merrily Watkins - I've developed a love for series, mostly because of the characters and the use of English folklore.
Count St. Germain Series (counting the Olivia books) - world's greatest vampire.