Depends on people's age! I was beyond childhood when that came out, but it was a game changer for me getting to play a FPS that was couch co-op. All FPS games to that point were mostly (all?) multiplayer competitive. PvE was always a solo experience until then!

Bungie gave me some of the best gaming childhood memories.

So much Halo. I was obsessed with it as a kid. The gameplay, the story... It was a SciFi gaming masterpiece. Super fond memories.

I'm also a HUGE fan of classic Marathon and Pathways Into Darkness. People don't give these games the credit they are due, and everyone should play them. You can find the Alephone engine, open-sourced by Bungie, and play all of the Marathon games and community scenarios completely free. A well-respected community member even ported Pathways to the engine.