Which early humanlike being had the largest brain size, making them advanced hunters and possibly resulting in the disappearance of the giant mastodons and mammoths?

The appropriate response is Homo sapiens. This is the binomial classification for the main surviving human species. Homo is the human sort, which likewise incorporates Neanderthals and numerous other wiped out types of primate; H. sapiens is the main surviving types of the sort Homo.