Fire.
Okay, it shouldn't be called invention, because it's a phenomenon, but if you stay just a few moments and think, you will agree that the human's ability to use the fire was one of the most important achievements of mankind. With it, our ancestors could cook better, could make TOOLS out of metals and survive in winter, eliminating the need of migrating to other warmer places. This way, communities were born and from there many things happened. Imagine the life without fire: no electricity, no heat in our comfortable houses, cooking would be a lot harder... so the conclusion is tha the fire is probably the greatest discovery of all times.
It's just an idea, you can criticize me...
Andrei