I’ll toss in my two cents…..
In my opinion the answer may be the 1963 Avanti R2…. Reason being, from my own experience, the scream coming from the supercharger is LOUD at anything above 3500 RPM… (the factory air filter setup is basically unsilenced)
Although the R3 Avanti is more powerful, it’s supercharger’s air filter is located in front of the radiator, which would quiet the supercharger noise compared to the R2 under hood setup.
As far as the R4’s engine song, I’ve ridden in a 425 horse 409 Chev (dual carbs) and although the two carbs, when opened up, were very loud, (sounded great in car!) I don’t think the sound was as ‘strong’ as a wound up Paxton.