Why is it that Harley Davidson is considered superior to other bike companies?

Thats no necessarily true, they are a factory production line product just like a car, or pickup from GM, Ford, etc. Where they differe is once they come off the line and on the floor. There are multiple ways to customize them, in fact HD makes millions of $ a year on selling custom parts piece by piece to give the user the feel of a custom bike. The reason they are the best is due to the that nastalgia associated with the name considering that other motorcyle companies have come and gone (HD being one of them) they fought through the rough times and once again are on top. They really a nice bike.