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.
Many games come close for me... Mass Effect, Zelda Link to the Past,, Factorio and Homeworld would round out my top 5 for instance. But System Shock 2 was my first immersive sim game. It started a love affair with the genre that I'm still trying to scratch 25 years later.
There were many spiritual successors, with Arkane's Prey being the closest approximation I've found. Nightdive just released the original System Shock's remake a month back, so the wait for them to announce they're giving SS2 the remake treatment is torture.