John - the US wasn't 'just the biggest'. It was simply the only Air Power that could go any place in the world (simultaneously)and deliver integrated Logistic, Tactical, and Strategic land based airpower as well as gobal sea based airpower - in overwhelming numbers and qualitative capability to any adversary.
The RAF and LW and VVS had niche superiority for some possible missions - unquestioned.
For the simplest of examples, while the RAF certainly had equivalency in Strategic capability in the ETO and MTO through 1944, where was it in comparable strength in the Far East? What was the delivery capacity of the RAF (absent US aircraft) to provide logistics support to bases loacated away from England? What distribution of airpower assets from GB occurred in the Pacific Theatre, or compare delivery of all aircraft types to the Soviets and Chinese and Commonwealth by the US.
The short answer is that natural resources, financial resources, skilled labor, manufacturing processes and floor space and tooling plus pilot/crew training capacity - combined with a full range of 'nearly the same to clearly better' products capable of attacking and supporting attacks on the Axis - directly or via proxy - anywhere in the world is what made US Airpower 'the most powerful'. If you wish to make your case that 'best' is not analogous to 'most powerful', explain away..