Well, to be the biggest in a Christmas tree story, the tree has to be physically large, perhaps taller or wider than any other in the area. Regarding the best, it's all about the details. The lights should be arranged in an appealing way, the tinsel should be just the right amount, and the ornaments should be carefully chosen. Maybe there are some that were made by local artists. Also, if the tree has a great backstory, like it was rescued from being cut down too early or it was planted by someone special long ago, that can make it the best in the story.
A big height can make a Christmas tree the biggest. And for the best part, it could be having really beautiful and unique decorations like hand - made ornaments that tell a story. Also, if it's a center of attraction and brings people together, like in a town square, that would qualify it as the best in a story.