I'm going to nominate the JT8D in the B727 as the most distinctive sound, due to the fact that when a 727 was approaching or departing Adelaide Airport (when such things were allowed) one didn't have to look out the window to identify the aircraft type
Recently heard the audio of a Bristol Centaurus engined Sea Fury idling and it sounds EXACTLY like a 1973 Torana GTR XU1 (race homologation special) with a 3.3l pushrod i6 and triple strombergs. EXACTLY. A big "cammy" high compression racing engine soundsound (with of course NO cams).
And I always get a laugh out of seeing people talking on the flight deck or cockpit of a piston engined AC in films. Even in the cargo area of a C130...