When I was younger, series I devoured were: The Saddle Club by Bonnie Bryant, Thoroughbred by Joanna Campbell, The Babysitters Club by Ann M. Martin and Sweet Valley High by Francine Pascal.
Sure, the subjects of the series have changed, but one thing seems to remain constant. All of these series have characters that make us care and want to read about them for multiple books. There's something about these books that just wouldn't work as well if the author wrote just one and left us hanging. Seriously, could Stephenie make Twilight as great if she had to cram all of Bella's ordeals into one book? Would Gemma in A Great and Terrible Beauty grow and discover her powers in book one? Doubtful. We need books two, three and so on to take us on the journeys of series characters.
So, what are your favorite past and/or present series? Why?