There are many works which are said the masterpiece of Japanese anime. However, if asked, ' what is the masterpiece?' by those who have just started to watch anime, I will recommend them this 1973 work, "Alps no shoujo Heidi (or, just 'Heidi')." It is based on a fiction 'Heidi' by Johanna Spyri, but the staffs of "Alps no shoujo Heidi" made some changes. They focused more on the way Heidi lived in Alps and in Germany, and on the relationships between characters, that is, the staff depicted the daily-lives of characters. This TV series was directed by Isao Takahata, famous for "Panda Kopanda," "Hotaruno haka" and so on, with Hayao Miyazaki, also famous for academy-award-winner "Spirited Away." It is not a well-known fact that Takahata and Miyazaki in their younger days made this work. The fact implies that "Alps no shoujo Heidi" is not only reputed as a well-made work, but it has a historical value. If you are interested especially in the history of Japanese anime, this title is must.
Speaking of its historical worth, I cannot overlook the work of Miyazaki. He helped Takahata well and drew all the layouts of this 52-episode anime series. This is a surprising fact.
(NOTE: in Japanese, 'layout' means 'the very important drawing in the cut, included background'; in anime animators do not depict backgrounds but layouts include background. Usually 30-minute long anime is composed of 300 cuts, so there are 300 layouts in one episode.)
Miyazaki's work is also found in many parts of this anime. He was also involved with direction. Therefore you can easily understand how great Miyazaki was, and still he is, by watching "Heidi."
There are more historical achievements in this anime, but I cannot write down all of them, so if you got interested, please check them.
Thanks to Miyazaki's effort, and other talented staffs including Yoshiyuki Tomino and Yoichi Kotabe, "Alps no shoujo Heidi" made a big success.
As for storyboard, it is still one of good models of anime today. "Heidi" could express Heidi's innocence, Peter's cheerfulness, Clara's agony, and Uncle's conversion. With precise backgrounds (thanks to a location hunting, which is the first one in Japan's TV anime history,) you can feel as if Heidi and other characters really smiled and cried.
As I said, the plot mainly traced the original, so I won't mention it in detail. Takahata and Miyazaki's success is that they succeeded to describe tiny changes of emotions with delicate directions, animations and the story. This success influenced (and, still influences) the later works, I mean, Japanese animators became to depict daily-lives of characters, not a Hollywood-like spectacle. In this sense, "Heidi" is the most important work of all.
Of course, you sometimes find its old-fashioned aspects, such as the designs of characters, recordings, and other technical constraints. However, this work for certain contains the essence of anime. You will soon notice that only a few titles can perfectly imitate "Heidi," and there are an amount of anime which aimed to "Heidi."
After 25 years, it is still a key work in Japan's unique anime world. It is when you notice the excellence of "Heidi" that the time you became interested in anime anew comes.