Gran Turismo Sport is pretty much just brand at this point (and believe me, it hurts me to say this).
Forza 7 is a really complete package that's a better game than it is a good simulator, but the handling isn't all THAT bad either. The game focuses on car count while maintaining a very high level of model quality throughout, and the campaign is built around collecting cars. If you want a successor to what Gran Turismo used to be, this is your game.
Project CARS 2, definitely my preferred choice out of the three. Fantastic driving simulator that focuses on realistically simulating pretty much every aspect that influences the handling of the car. The campaign is built more around simulating motorsport events than car collection, and it's pretty complete in terms of content. There's plenty of cars (not as many as Forza) all of them modeled with insane attention to detail, especially in the mechanical components. Yes, even the ones you don't normally see. It's also got the most real-life tracks and they are beautifully made.
In a nutshell: If you want what Gran Turismo should be today but isn't, Forza 7 is your choice. If you want the most advanced motorsport simulation, the most realistic handling and the best graphics, go for Project CARS 2. I would get both tbh.
Full disclosure: I helped fund the original Project CARS (totally would have funded PCARS 2 but sadly I missed the boat), and I ended up making a pretty good profit off the investment too even though that's not why I invested in its development.
