It would help to narrow down context a bit, but I’m inclined to say the spear, because it was one of humankind’s main weapons for most of history and allowed us to avoid being constantly owned by big cats and other land predators. Alternatively, you could say the invention of clothing or coming up with reliable methods of starting fires.

As you can tell, I tend to interpret this question in a way that emphasizes early inventions for their role in helping us get “off the ground” in the first place, partly on the reasoning that those things were requisite for everything thereafter.