The question is too vague. I will instead focus on what is the most important sim in sim racing history. And to me the title goes ex-aequo to Revs from Geoff Crammond (1984), the first 3D racing sim, and to Indianapolis 500 from Dave Kaemmer's Papyrus (1989), the first convincing 3D racing sim with decent AI, car setup, analog steering (with the mouse), replay etc. These two guys laid the foundations for racing sims to come, and Kaemmer is still active with iRacing. After these, surely F1 GP from Crammond and GPL from Kaemmer. The one I love the most is AC, it has so much variety and it is not only a racing sim.