I would have said the F1 Ferrari V12 engine sound aswell but I felt there was no need with it already being mentioned a few times. Gorgeous sound might I add!
I physically start shaking when I hear this.... just sends shivers down my spine and makes life seem more... worthwhile. I'd love to hear it in person though.
Another vote for the Quattro, and all utterly mental rally cars. In my mind, a race car engine should sound like giant hornets trapped inside a burning fireworks factory--I don't want my engine to sing, I want it to sound like it's going to explode at any second with the sheer extremity of what it's being demanded to do. And that wastegate noise that the Quattro and a few other cars have--I have no idea what causes them to sound that way but I love it.
But I'm also very fond of the BMW M1Procar. More of a "pure" engine sound.
From all the cars I've driven (nothing fancy really), the little stock Suzuki Swift '95 1-litre 3-cylinders had the most exciting sound - how ironic, eh?