LOL, I can't decide haha.
This doesn't do it justice, but the small displacement Ferrari V12s with no muffler are incredible:
isn't quite stock but I couldn't find a good soundclip from a stock srt8. But it sounds like it's going to rip your face off- I love it.
I can't find a good clip of an S2000 really going for it, but at 8000 rpm they sound like an F1 car. They are just ok at idle.
I'm only looking at cars I've actually heard in real life, youtube sound is sort of mediocre. I think the old Ferrari is my pick, I've never heard an engine sound even close to like that since. It was so distinctive.