You could say the same for the V-1710 though. What would have happened in the pacific without P-40s or P-38s (and to a lesser extent the P-39). For the R-2800 there's the F6F and F4U which the navy would have been crippled without. Immagine the F4F trying to continue on the front line in 1944 (or F2As for that matter)... Or if there was no P-47, verry bad for the US in the early days in the ETO.
Also I cant see why the B-29 couldn't use an uprated R-2800 instead of its Cyclones, the 2800-57C put out 2,800 hp, and though it had some mechanical problems early on, I doubt it would be more troublesome than the 3350's. Plus the B-29's R-3350-23 engines were rated at only 2,200 hp and many models of the 2800 met or exceded that. Even the earlier 2800s put out 2,000-2,100 hp.
Not saying the Merlin was any less important, but it's really hard to say it's the MOST important... Besides if they'd been able to use a turbo on the P-51 the Merlin-61 would have been unnecessary. (the V-1710 with auxiliary supercharger would have worked like in the P-63 which had a slightly higher ceiling than the P-51, though this wasn't ready 'till 1943; as a plus the P-63's engine had a higher millitary power than the Merlin 61, 1600 hp and 1800 hp with water injection, and it was lighter)
-Most important UK piston engine is the Merlin Hands down, though.
-Most important USN engine is the R-2800 (though the TBD, and F4F's 1830 was also quite important, though mostly early in the war) The R-1820 of the Dauntless and F2A was also quite important. (The Navy could have done fine if they had F2A2's in quantity instead of Wildcats IMHO)
-USAAF: not so clear, the V-1710 is up there (definitely the most important Liquid-cooled engine), but the 2800 isn't too far behind, and the merlin is there too. Though without It we wouldn't have had an effective fighter to stave off the Japanese in the early says of WWII, both the P-40 and P-39 are highly insremental in this, and later the P-38. What is the P-40 without an Allison, just a P-36 with a stronger armament. And don't say the merlin could be used instead, since though it was later substituted in some, there were nowhere near enough merlins for this in 1940, and the Packard V-1650 wasn't even run untill 1941. Plus ther were all the engines used for bombers and attack a/c.
-And of course the C-47 used the 1830, but I think there were alternatives.
And even so there was no fundemental flaw in the V-1710 that made it less than the Merlin (indeed the early models of each were pretty equal), but it was a USAAC philosophy that gave the bad altitude performance seen in the P-40, P-39, and P-51A.
"The most serious change, however, was the elimination of the turbosupercharger, and its replacement by a single-stage geared supercharger. This change was a result of a shift in philosophy on the part of the USAAC. The USAAC believed that the widths of the Atlantic and Pacific Oceans made the USA virtually immune from high-altitude attack by enemy bombers. Therefore, the development of high-altitude interceptors was curtailed in favor of strike fighters optimized for low-level close support. The 1150 hp V-1710-17 (E2) of the XP-39 was replaced by a V-1710-37 (E5) engine rated at an altitude of 13,300 feet."
"The Allison V-1710 aircraft engine was the only indigenous US-developed V-12 liquid-cooled engine to see service during WWII. A sturdy and trustworthy design, it unfortunately lacked an advanced and efficient mechanical centrifugal supercharger. Although versions with a turbosupercharger did give excellent performance at high altitude in the Lockheed P-38 Lightning, the turbosupercharger was only fitted to experimental single seat fighters, with the same excellent results. The preference for turbosuperchargers, arguably to the neglect of mechanical supercharger development, reflected US Army philosophy, and not the inherent qualities of the Allison engine...The Army had earlier decided to concentrate on turbosuperchargers for high altitude boost, believing that further development of mechanical turbochargers would allow their engines to outperform European rivals using superchargers. Turbosuperchargers are powered by the engine exhaust and so do not draw power from the engine, whereas superchargers are connected directly by gears to the engine crankshaft. Superchargers as a result require increasing proportions of engine power as altitude increases (the two-stage supercharger of the Merlin 60 series engines consumed some 230-280 horsepower at 30,000 ft). General Electric was the sole source for research and production of American turbosuperchargers...However, mating the turbosupercharger with the Allison V-1710 proved to be far more problematic. As a result, designers of the fighter planes that utilized the V-1710 were invariably forced to choose between the poor high altitude performance of the V-1710 versus the increased problems brought on by addition of the turbosupercharger. The fates of all of the V-1710 powered fighters of World War II would thus hinge on that choice...The original XP-39 was built with a turbosupercharged V-1710. Numerous changes were made to the design of the production version (after a review by aerodynamicists at Langley Field), including a decision to drop the turbosupercharger. This decision came out of a combination of the severe teething problems encountered with the turbosupercharger mated to the V-1710, as well as the belief of the Army at that time that high altitude performance was not necessary in a fighter plane. The P-39 was thus stuck with poor high altitude performance and proved unsuitable for the air war in Western Europe, which was largely conducted at high altitudes. The P-39 was rejected by the British, used briefly by the U.S. in the early Pacific air war, and then was exiled by the USAAF to the Soviet Union under the Lend Lease program. The Soviets were still able to make good use of P-39s because of its excellent maneuverability and because the air war on the Eastern Front in Europe was largely short ranged, tactical, and conducted at lower altitudes."