Auto Racing Types
Quite frankly there is no "most difficult" They all require similar but slightly different skill sets.
Open wheel racing (Formula 1, Indy, GP2 requires the best physical conditioning full stop, and incredible precision. That said, the engineering might of the team, make the cars easier to setup and develop, and while the cars limits are high; they are well designed and harken back to karting skill sets. They are usually somewhat natural to drive for a racer. Here your physical conditioning, commitment, precision, and cockpit management (adjusting settings by your crew's orders) are most important.
Nascar: While on the surface looks very easy, it is actually very complicated. While the teams use full data systems during testing they are banned on weekends. This is important as due to the high speed, and long duration corners, car setup is very crucial. Here is where we can see the feedback, and communication skills of the driver. You must also be very precise as where you lift off the gas, has ramifications for your splitter height entering the corner (and thus chassis balance). In addition, you spend the majority of the race, in very heavy and semi equal traffic creating a stressful condition for the driver. The skill sets to be successful here is Feedback, communication, racecraft (how to pass and be passed) and precision.
Touring Cars. A unique form of racing. Here you are racing cars that start as street cars and racing generally short (though some series run endurance races). While these cars are not very fast, or physically demanding; they are often very difficult to drive on the limit. Their street car heritage requires compromises in the setup so a driver must ring the neck of the car more often. Also the racing is usually very close like nascar, and the heavy cars (sometimes with large amounts of power) can require tire management to be key. In this form of racing Driver feedback is key (to figure out how to setup the car) race craft, and car control. (ability to slide and catch the car