I think the most(ly) diverging sound might catch most attention and out of this conclusion is the 5-bladed Griffon Spitfire among My better experiences!
But the radials do have an impressive sound, however the Catalina and the B-17 at the last airshow I visited was a huge disapointment. I guess that the engines wasn't pushed much :-/
To Armand, if you want a loud roaring engine, Heavy bombers won't provide that, they have a rumbling humming sound that I would say is quite relaxing. I prefer the high octane make your ear ring Pratt & Whitney's and the Merlins. The Griffon powered Seafire and Fairey Firefly have a very unique, distinctive engine sound that isn't as smooth sounding as a Merlin but sound much more powerful.
Something I didn't point out is that some aircraft that share the same engines as others have a different sound. The Merlin in a Mustang sounds slightly louder and more powerful than in a spitfire. The best example of this is between the F8F Bearcat and the F7F tigercat. Both aircraft have Pratt & Whitney JR-2800 double wasp radial engines, but they don't share the same audio effects. The Bearcat has a quick timeframe where the engine is heard where it is very loud, or when racing it has a rather quiet whistle too it, or a rumble. The F7F has a serious build up of noise that sounds like a roar. It's rather hard to explain but anyone that has heard any of the two will understand what I'm saying.