The Four Largely Popular thrash metal bands that formed in the 1980's. These bands dominated the thrash scene.
They were,Metallica,Megadeth,Slayer and Anthrax.
runners up include-
Exodus,Death Angel,Overkill, destruction and Kreator
the big four of thrash were influenced by hardcore music and NWOBHM and went on to inspire nearly all of todays metal. Slayer Alone is said to be one of the forefathers of death metal.