Given the amount that its' sound is used as a 'catch all' jet fighter engine sound by 'foley' sound engineers in film and TV, it has to be the RR/Bristol Pegasus.
Personally, it is a close run thing between the RR Griffon and Merlin. The Griffon just shades it; it just sounds like raw power, especially in a late mark Spitfire. The noise as it comes down off a steep turn, with the supercharger whine is something to behold.
A special mention must go to the Allison T56-A, but that is more to do with it being evocative of where I live (and grew up...)