Start with Terra Incognita or The Link . If you want the "essential" classic: Start with From Mars to Sirius . If you like Lamb of God/Meshuggah: Start with The Way of All Flesh . If you are new to metal or prefer modern rock: Start with Magma or Fortitude .
Where From Mars to Sirius was about hope and space, The Way of All Flesh is about death and decay. It is arguably their most technically difficult album. The riffs are jagged, the tempos are manic, and the lyrics stare directly into the void of mortality. Gojira Discography