Go allllllllll the way back to 1985. I had just started dabbling into hardcore with Venom (whom Metallica used to open up for on tour, ya it's true). I went over to my friends house and he put this album on while he went to take a shower before we went out partying. This song was first of course, and it completely blew me away, changed my musical tastes forever. Not to mention it was followed by Ride the Lightning and For Whom the Bell Tolls.
By the time he got back and I had listened to side 1, my jaw was on the floor. I had lived, ate and slept Black Sabbath until then, which were the heaviest ever by far, even up to that point. But this was the beginning of a new generation of metal.
Metallica - Fight Fire With Fire (Live in Mexico City) [Orgullo, Pasión, y Gloria] - YouTube