I love this question, but its so hard to choose! I also have choosen my top three series. I also have a favorite book that tops all of these but I've decided to go with series. My all time favorite book it The Princess Bride. This is also one of the few movies where I am not disappointed in how they did it. I love both!
The Hitchhiker's Guide to the Galaxy most definitely holds the top spot. I have a great love of ridiculous British humor. These books kept me laughing and entertained. I've been meaning to go back and re-read them. So much I have forgotten about them. Douglas Adams really did a phenomenal job of creating a really different and eccentric world. This is a world I would love to live in. One where a Towel if your most prized possession and you can put a slug in your ear and be able to understand everyone. Where 42 is the answer and you don't get to know the question! There's romance, adventure, and just about anything you could ever thing of.