I had to think a bit about this topic from Long and Short Reviews. What book series do I actually read? I've read quite a few YA dystopian series but wouldn't consider most of them to be among the best I've read - I always had some sort of problem with them.
The first series I could think of was Philippa Gregory's Tudor series. I don't think any of them are as good as "The Other Boleyn Girl", but I love that period in history (and I am aware there are disagreements as to the accuracy of Gregory's books) and I felt like she made the period come alive. It takes a lot to make me interested in Anne Boleyn after having read a dozen different versions of her story.
Similarly I loves Suzannah Dunn's series of Tudor novels. Written in a more modern style, they capture the mood well without getting caught up in language.
Isaac Asimov's Foundation series is always a favourite. I always admire people who can write good science fiction, since I love the idea of complex world-building but never have the nerve to try it myself.
And I couldn't leave it without mentioning one of my own. My trilogy "I Heard Your Voice", "Tamar Rising" and the upcoming "Spiritwalker" were some of my favourite stories to write. I love writing paranormal romance, and paranormal investigator Tamar Steele was an amazing character.
So which series have you loved and why?