My favorite has to be the Pratt & Whitney R-2800 radial engine.
I was about two hundred yards from a P-47D-40 idling near a hangar at Hooks airport (just a few miles from my house)... That was the loudest thing I've ever heard (and I had the windows on my car rolled up!).
And a month or two ago while I was riding my bike, a P-47 flew overhead. I might've been the same one, though I'm not too sure. It's kind of difficult to make out a paintjob on a plane that's 2,000 feet up in the air.

Anyway, the one that flew overhead was running at about 70% throttle. The "hum" of the R-2800 was very noticeable, though it was still nothing compared to what it could be on full throttle (it was a low-frequency hum rather than a scream).
And a few years ago at Hooks airport, I was very close to a TBM-3 Avenger (I actually touched it a few times). It gave me a bit of a scare when the guy who was maintaining it decided to start it up, seeing as I was only about twenty feet away from it. The engine had the same sound as the P-47's, though it wasn't quite as chunky.
Yeah, I like piston-engined aircraft, too.
