Electricity.
Without it, we would be powerless...(my little joke).
Allow me to demonstrate: Black out an entire metropolis. Observe the chaos.
QED!
As for fire, indeed you can't argue with it...Fire has limited debating skills and will likely win and burn us. However, we didn't exactly invent fire, we learned to harness it and use it (and only so much).
I actually have a more systemic vision of the world. I don't think a single invention made us strong. I thing it is a system of inventions which made us strong. 1 invention branched out and spawned multiple applications throughout multiple fields (Laser, ...), and when combined with other inventions, resulted in another invention...
The Internet may turn up to be the greatest invention of OUR lifetime. Then again the Internet is more a SYSTEM of inventions than a single one. It is also a mighty enabler of more technological, scientific and medical advances, partly because it allows to share knowledge instantly.