Roads or sanitation.
Fire.
(I know it occurs naturally but harnessing it and using it as a tool is a unique achievement of the human race.)
It preceeded the creation of every tool, chemical process, it defended us and shelterd us (against wildlife and cold even infections trough cooking), combustion is literally the basis of every single modern mechanical or electrical system. We used it for conquest and for defense. We built our entire civilization with fire.
Agriculture: it let previously nomadic people settle.
Written language: needs no explanation.
I couldn't decide.
the internet. it has not yet reached its fullest potential and yet has made much contribution. the way to technological progress is mass communication.