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.
"They could see King Caspian raising his hand to bless his son. And everyone cheered, but it was a half-hearted cheer, for they all felt that something was going wrong. Then suddenly the King's head fell back upon his pillows, the musicians stopped and there was a dead silence. The Prince, kneeling by the King's bed, laid down his head upon it and wept."