I'd have to cast my vote for MotoGP motorcycle racing or almost any of the AMA races. If you all have never watched a MotoGP race I would highly recommend it. It's the most exciting racing I've ever seen. Think of F1 speeds, near F1 cornering speeds, and only leather to protect you, and you're still racing 6 inches from the next guy and 0 inches from the ground in the corners. The horsepower to weight ratio on those bikes is incredible. It's like racing a 1500 hp Porshe, but riding on top of it
The Isle of Man is just crazy, I'd say it's too dangerous, but amazing to watch.