There's nothing I love more than knowing that the story I'm enjoying will continue long after I finish the book I'm reading. A series, too, gives the author time to flesh out characters, show their depth, as well as have them change over a protracted period of time. Also, larger story arcs can be explored that extend over a number of books, adding to the overall complexity of the world in which the story occurs.
A few of my favorite series that have maximized world and character development would include:
Troubleshooters series by Suzanne Brockmann
PsyChangeling series by Nalini Singh
Nightrunners series by Lynn Flewelling
Adrien English Mysteries by Josh Lanyon
Now, I understand the whole saw about how it's better to leave people wanting more, but here are a few books that I really wish were series:
Sunshine by Robin McKinley
The Graveyard Book by Neil Gaiman
This story of the orphan Nobody Owens (Bod, for short) who is raised, protected, and loved by the ghostly inhabitants of the the graveyard is sweet, dark, and lovely. It's reminiscent of Tim Burton's movie, The Nightmare Before Christmas, with its twisted sense of humor. By the end, I had a lump in my throat, and I wanted to follow Bod on his further adventures.
Hemovore by Jordan Castillo Price
Do you like series, or do you prefer stand alone novels? Is there a book you wish was a series? Or is there a series that would have been better left as a stand alone?