Mustaine was such a factory back then that he pretty much threw off enough riffs to fill his own albums, and a significant portion of Metallica's early ones (check the McGovney tapes : easy tell on who wrote what - Hetfield's riffs tend to return to the open E string, Mustaine's rarely do). And the thing is they werent verse-chorus-lala, it's four or five of the most crushing riffs imaginable per song. The deep cuts on those early Megadeth albums are still unbelievably hard forty years on. And this was really the best lineup with Samuelsson... listen to some of his drum-isolation tracks. He played like nobody else in metal - probably because he was playing jazz. He syncopates over the riff and you don't really notice, but it adds a power to it that other bands didn't have. Even without any other instruments or vocals, you know right where you are in the song because Gar is ALSO playing riff. Almost like Ginger Baker or Phil Ehart of Kansas.