For me, I believe it would have to be the resurrection of Christ, because of what it did for mankind (yes, that includes women too). I believe this to be true, and I realize that most would likely not agree. God gave each of us free choice, so do exactly that, and chose.
From a strictly historical perspective, although I agree, the taming of fire was certainly a milestone, I believe that invention of the wheel could also be considered humanity's greatest achievement. Regardless, with niether fire nor the wheel, and agriculture for that matter, no human would have ever stepped foot on the moon, much less drive to the convenience store to buy a loaf of bread!