For me, it was Grand Prix 2 / Grand Prix 3 (Geoff Crammond again). The handling of the car with driver aids off and a steering wheel / pedal set in use was, in my experience, unrivalled. It felt like a real simulation, not an arcade thrill. Add to that the active modding community (of which I was a part) and it was just excellent. I still really miss designing my own tracks and would love for a game to give me that ability again (on PC).
Or maybe I’m missing out and just don’t realise something already does?