I don't read much Scifi/Fantasy anymore, but when I did, it was The Wheel of Time series by Robert Jordan. It's fantastically written, and there are over 14 books now, I think. Unfortunately, Jordan died a couple years ago, so I don't know what they're going to do to finish the series.
The other series I couldn't put down was all of the Dragonlance books! Mainly, I preferred the ones by Margaret Weiss and Tracey Hickman (they co-wrote them). =)
I loved these two series because they are imaginative, well-written, the plots are perfect (LOL. okay, so no plot is perfect, but you get what I mean), and of course the fact that they are many books in a series definitely helps.
I hope you try these two out. Happy reading! =)