Whether you started with a 2600 and a joystick in your hand, an N64 with a blistered palm or building your first PC in your teens, what is that one video game you've played at some point that to this day sits at the top of your list.
If you haven't played it, don't read anything about it, just go in blind. It's an amazing experience that I wish I could play again for the first time.
Some other favorites worth mentioning:
Minecraft
Celeste
Doom 2016
What Remains of Edith Finch
Titanfall 2