Probably metallurgy--the discovery that mixing iron with a little tin will make bronze and that mixing iron with a little carbon will make steel. Warfare, conquest, the migration of tribes, and the creation of nation-states and empires were based off of those discoveries for about 2,500 years. Elecrticity has only been really used for about 140 years and can be wiped out by a well-placed solar flare. Modern medicine has only been around more or less for 200 years and is easily subject to being lost to anti-intellectuallism and religion. Nuclear energy, around for only 70 years, can cause mass extinctions and the end of civilization.