My family really enjoys this series, so much so that we named a child after the hero. We recently finished reading the entire series aloud; our 10 year old and 8 year old were entralled, and the 4 year old was really out of the loop. I recommend the books.
Book 4 of the Chronicles of Prydain is the most powerful of the 5 books. The overall series follows the life of a young man who starts as an orphan aspiring to be an Assistant Pig Keeper, and who ends up as the country's High King. In this volume, he has matured enough to have taken the grandiose dreams out of his head. No longer does he wish to be a fierce fighter, or the greatest hero of all time. He does not, though, know what he is really meant to be. He travels the country looking for a place to fit in. He meets all manner of everyday people, and comes to respect them each for their own gifts. In this travel, as he becomes more certain he'll never find his true place, he is actually gaining the perspective (and connections) that will allow him to be the only one who can rule the country fairly. The reader, while joining Taran on this quest, also gets to explore the value in the everyday. A young mind often overlooks the shepherd, and focuses on the knight. In this book, one finds that the shepherd has a job that requires strength, skill, and courage. Tending life, in fact, is far more valuable than taking it.
Lloyd Alexander is one of the greatest storytellers of all time. He was able to construct this story so that it is accessible to the young reader, but is also full enough to engage an older audience. This book is meant to be read in the series, but you may find yourself going back and reading it again, just for a perspective refresher! I can't recommend this book highly enough.
If I had to pick a favorite from this series, Taran Wanderer would probably be it. It's a quintessential "finding oneself" novel in the fantasy genre. Taran goes from one interest to another and learns plenty about himself in the process, of course.