As crazy as it sounds, Forza is probably the best exclusive series Xbox could have ever gotten. Halo is the most popular, but no other franchise pushes the boundaries like Forza.
When you're making a full out simulator, you have to give it everything you got. I watch Formula 1 events and Le Mans on TV, and there are constantly references to the game. It's not a game, but a professional simulator.
One of the most important things when driving a real car is being able to look out the windows to see your surroundings and the turns you are approaching. Forza is the only driving simulator to let you do that.
Check it out. This is what a simulation should be like: Forza 4 CSR Elite Drift Fujimi Kaido B Reverse - YouTube
The way everything handles is how it would in real life, with force-feedback not only on the steering wheel, but also on the pedals too!