Who is Metallica's biggest influence?

I'm curious, could you elaborate on who you believe to be the most significant influence on the band Metallica? Have they cited any particular artist or genre as a major source of inspiration for their music? And how has this influence manifested in their work, both in terms of their sound and their lyrics? I'm particularly interested in understanding the depth and breadth of this influence, and how it has helped shape Metallica's unique identity in the world of rock music.