Absolutely the USA.
Remove some of the WT gates (and lose the entire Monroe Doctrine tree altogether - it's just bad), and give the USA the ability to respond to certain specific actions by the axis powers. Specifically, the USA should be able to respond to Germany capitulating allied majors and declaring on the soviets. In particular, a successful operation Sealion should allow the USA to quickly bolster its support of the UK (more volunteers + quickly increasing war support). Taking London should basically result in an insta-DOW to prevent the Allies faction from surrendering if the UK capitulates. Maybe allow the USA to change laws to War Economy/Total Mobilization early?
At the very least, I'd like to see an event for the USA intervening if Germany lands in Canada. In the era of the Monroe doctrine, it is absolutely nuts that the dominant strategy for taking out the USA as Germany is to Peace Conference annex Canada before the USA even joins, and then line up on their border. That just strains credibility: for one thing, Canada wouldn't just accept being annexed, for another - the USA would have intervened early if Germany had tried to cross the Atlantic in any fashion.
From the Axis perspective, the USA should be the ultimate end boss (certainly for the Allies, perhaps for the entire game). Successfully mounting an invasion of the continental United States should take extensive planning and coordination between your Army, Navy (insofar as the Navy is a functional part of this game) and Air forces. As it is, you can line up on America's doorstep without even getting DOW'd.
From the Allies perspective, the USA would like to play this game too, thanks. Every game I've ever tried as them has ended with me giving up in despair as the UK and SU trash Germany (and even if the SU loses, they've weakened Germany to the point where they just fall over as soon as the first landings happen).
To me, making the historical path for the USA fun is more significant than the ahistorical path. Frankly, a fascist USA is enough of a juggernaut already that they don't even need a focus tree + 99% of the time, they're going to follow the historical path.
France takes a distant second. They're not great right now, but they at least get to participate in the war.