Hoo boy what a question.
TL;DR: I would say give at least one song from each album a try since they have a good amount of variety in their sound from one album to the next. I recommend:
Terra Incognita: “Clone”
The Link: “Remembrance”
From Mars to Sirius: “The Heaviest Matter of the Universe”
The Way of All Flesh: “The Art of Dying”
L’Enfant Sauvage: “Planned Obsolescence”
Magma: “Magma”
Fortitude: “Fortitude” followed by “The Chant” (they go together)
Otherwise,
If you like heavy metal, Terra Incognita is great. “Clone” and “Blow Me Away You (Niverse)” get stuck in my head a lot. “Planned Obsolescence” from L’Enfant Sauvage (one of my favorite songs ever) is about as in-your-face “fuck off, capitalism” as a song can get.
If you like environmentalism in music, the theme persists throughout their discography but From Mars to Sirius is chock full of it. “Ocean Planet” builds to one of my favorite lines in any song ever with a painfully dissonant melody at the very end of the song. It’s also hard to ignore “Flying Whales” with all the memes and the song is written from the perspective of whales looking down in sorrowful pity on how humanity treats their shared home, plus the song has a wonderful chorus. And I can’t not mention “Global Warming.” It might just be Gojira’s magnum opus imo. “Toxic Garbage Island” is a really cool rhythm-heavy song about treating the planet like a giant trash can with one of the most emotional finales of any song I know.
“The Art of Dying” is an incredible song about what it means to die well. The music is incomparable, and it pairs really well with “The Way of All Flesh” which is an artistic audible rendering of the experience of actually dying - only listen to it if you plan to listen to all 17 minutes of it.
If you need something more encouraging,
“Born In Winter” has been my track of choice many a late night. “New Found” is also shockingly uplifting compared to the rest of their music.
Truthfully, I love all of their music and I normally will turn on an album and listen to it start-to-finish. I most commonly do that with their album Magma because it’s extremely well paced musically and emotionally. While in the process of writing it, Joe and Mario Duplantier’s (vocalist/guitar and the drummer) mother died and you can definitely feel it. The album is mostly about the way that strong human emotions can feel like Magma boiling inside of you, and I think it’s extremely well conveyed. I go back and forth between thinking “Global Warming” and the entire album Magma are their greatest work.