There will be spoilers for the game and the DLC in my text and the comments
If you don’t want to be spoiled, play the game first, it’s a really really nice game that’s best when discovered
My favourite moment for the game (as for most players), was when I fetched the Warp Core and had to go to the Nomai Ship. The music made my heart beat so much, and the trip seemed so friggin long.
But my best discovery within the game is understanding how simply turning off the flashlight would be enough to make the quantum objects move. It was so satisfying discovering it for the first time.
In the expansion, I accidentally discovered what happens when you put your lantern down and walk away… That one was easily the biggest suprise out of the whole experience!