As Yumid said, it isn't one thing but a combination of all things. Personally I'd say the biggest key to making a good drop is simply contrast, that whatever precedes the drop is specifically designed to not have whatever is so cool about the drop. Perhaps the buildup has much less bass, smaller sounds, and then when you get to that empty measure before everything comes back in make it reeeaallllly empty so the return of all the huge parts of the track hits you in the face like a ton of bricks. The more you can say, "It was this way, and now it's the opposite of that." the better.
Beyond that, yeah, good sound design and melody are both super important.