It'd be the one with the most air moved. The biggest propellers turning the fastest, so I'm guessing that it'd be the fastest plane within the largest weight class.
So... a gunship that moves fast? I'm not really good with aircraft, so I can't quite name one, and I'm not really sure if there was ever a WWiI plane that would even fit the bill, or if it'd have to go to a strategic bomber or something like it. There's also the complication of more propellers versus fewer with more surface area, positioning, windflow, etc. It quickly turns into an aerospace science question that I'm sure some people somewhere get paid a ton of money to actually calculate.