The 18m estimate is for the very biggest females; most females got to 15-16m when fully grown.
Aside from size, meg deserves attention for just how long it existed (most of the Miocene plus around one and a half million years in the Early Pliocene). It existed for nearly 20 million years, when most apex predators only last for a couple million years, five million if especially successful. And it wasn’t like it had free reign because it had no competion; the opposite in fact, it had a huge array of cetaceans (including perhaps the ultimate raptorial cetacean) and other sharks that competed with it (the Late Miocene Pacific between 10-8MYA in particular has to rank as the most dangerous ocean ever). To top it off megalodon even managed to outlast all of its cetacean competitors (orcas wouldn't become large raptorial predators until later) and some of its fellow large predatory sharks it competed with.