YouTube channel Ruthless Metal believes they have uncovered the most recycled riff in all of metal. The channel dubs the riff the "Two Minutes to Midnight" riff, as they all sound a bit like the opening to that song (and it's the most popular example).
Ruthless Metal writes "The '2 Minutes to Midnight' riff by Iron Maiden might just be the most recycled riff in heavy metal history. It probably originated in the seventies and it was popularized by Riot and later used by Venom, Saxon, Judas Priest and Mercyful Fate amongst others. So in today's video, we're going to explore the most recycled riff in heavy metal. Enjoy!"
I can see where Ruthless Metal is coming from and I don't totally disagree. The examples in the video are definitely similar, but it's worth noting that the "Two Minutes to Midnight" riff is based off an open A string and just kinda plays around with shapes on the D and G strings. So it makes sense a lot of riffs sound like it.