Not sure I have one in particular that defined it, although I do have four that instantly come to mind: Heart of Darkness, Tomb Raider 3 (both to a lesser extent), Deus Ex, and Thief 2.
I was shit at all of them, but I think the feeling of just missing out on being able to experience DX and Thief 2 properly when they first came out is what has led to my absolute need for developers to start making immersive-sims again.
It does kind of me put at odds with most people who seem to have been fed Nintendo's stuff through a bloody IV drip.