What do you think is the most important technological advancement of the Indus people and why?

I'd love to say the button, which they did discover, because I like it when my pants stay on. But, the Indus people are largely credited with the invention of the ruler/measuring rod. Standardizing length allows for even building, cutting, and the like. It allows for measured clothing. It allows, at base, a common understanding of how big something is and should be.