Nothing - Meshuggah
It wasn't their first album by any means, but I think it was the most influential on metal music, it kind of spawned its own sub-genre (djent-core?). On this album they really honed in on the sound they made steps toward with Chaosphere, and it influenced the sound of metal moving forward into the late 00s and 2010s.
I haven't really been into metal as much as I was back when this came out, but it was really interesting to see this swedish metal band I randomly heard about in an AOL chatroom take off and become hugely popular and influential.
Yeah I was debating between saying Chaosphere or Nothing, I can definitely see your argument as well. Really cool band with incredible musicians all around.