Do you have a strong preference?
F1 by far.
Yes, I realize the actual racing can be somewhat boring at times with so much emphasis put on qualifying. From a technical/engineering standpoint, F1 is second to none. Approximately 750 HP from a 2.4 litre V8 revving at 18,000 RPM, accelerate to 100 MPH in a blink of an eye, transmissions that shift in milliseconds and cars that stick to the road like glue is some feat of engineering.
The Indy car series is quite competitive and can be fun to watch on a street or road course. Ovals are bore.
As for NASCAR, what a damn joke, hell they still use carburettors. WTF is wrong with THAT picture?