It's just about completely faded from cultural memory, but this animated TV movie from 1982 is at the top of my childhood favorites. Other Rankin/Bass productions, like The Last Unicorn, might have more popularity, but I think this takes first place.

The mixing of the real world and magical realms is quite unique and not copied all that much. If you're a Ghibli fan or just into anime generally, this could be a great recommendation for ya. Need more convincing? Just watch Ommadon's speech! Every word out his mouth is coated in delicious villainy.