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I have the best answer to this question.
The best engine of the war, was the engine that kept running long eneogh to get you back to base....
Don't forget the Merlin... Spitfire, Lancaster, Hurricane, Mosquito, Mustang etc
It's probably more interesting to look not at the engine itself, but at the aircraft it enabled to be built, and the effect they had on the War.
That's why there can be only one answer - the Rolls Royce Merlin.
Look at the list of aircraft: Spitfire, Hurricane, Mustang, Mosquito, Lancaster, to name but a few. If you want more, you could go on with the Kittyhawk MkII, Fulmar, Battle, Defiant, Barracuda, Beaufighter MkII, York, Whitley, Halifax, Wellington MkII, VI, etc, etc... True, not all were overwhelming successes, although most were.
All in all, when you look at it from that point of view, there was no single more important engine.
The Klimov, the ASh-82 and Mikulin engine series were vital as they powered the Yak, La, and MiG fighter series and the Il2.