The Big Four

In The Beginning Metal was... And then Metal got better! For Thrash was invented!

Metallica
Metallica started the Thrash movement, therefore seemingly created Heavy Metal and are GODS! However, the term "Thrash" didn't come from them... it came from...

Anthrax
Anthrax, technically the fourth of the bands to emerge, but for the sake of the name of Thrash coming to be, they are placed second. "Thrash" came from a song title called "Metal Thrashing Mad", and thus the name THRASH METAL was born!

Megadeth
Metallica kicked a guy named Dave Mustaine out of the band and he went on to form Megedeth, the 2nd best band of the BIG 4! Course... every decent Metallica song he claimed they stole... notice when The Black Album came out... suddenly Mustaine shuts up about them stealing songs... whatever. Megadeth Rocks and deserves all respect!

Slayer
Slayer, carved into many a forearm of stupid kids. Slayer kind of gave the genre a bad name (Not entirely their fault), much the same way Bon Jovi gives love a bad name. With Slayer, a bunch of kids started being stupid and parents got mad. And since NO ONE can take responsibility for their own actions these days... poor Slayer gets the crap. They still rock and are totally innocent.