The main reason I loved this weapon was because it had so many forms.
One of things I loved most about Inuyasha was how Tessaiga changed throughout the series.
It was a strong sword to begin with, and then became stronger as Inuyasha spent time leveling it up into an ultimate weapon. I like how each time it leveled up it obtained a new attack.
I just thought it was an awesome weapon, and Rumiko Takahashi did a really good job of creating a weapon that could develop, instead of Inuyasha having magical powers that became stronger.