Well, the Great Valley Adventure is in truth the first sequel, and first movie in the style of all the sequels that made us fall in love with the franchise.
It was actually the fourth movie I've watched after the original, III and IV. I found it at the local library when I was about ten, I was so happy I even cried a little bit when I saw it.
Its animation and storytelling set the bases for all the sequels who followed, and it rekindled the franchise, launching it forward.
The Time of the Great Giving was my least liked sequel when I was growing up, it’s a bit slow, and it lacks the action of II and the wonder and high stakes of Journey through the Mists. It has its merits, but it just didn’t have as big an impact on the franchise as the first sequel.