I’d say the Soviets and absolutely not Japan or the UK.
I’d say absolutely not Germany, b/c they will get attacked by 3 powers, at least if the allies conduct an efficient strategy 🙂
With the US a newb can get to feeling left out by lots of waiting to move something or for a chance to even just roll some dice. And they really need to deal with all of the units and trying to manage some sort of transport system. The US might be best for the team in that game as they supposedly can’t make big blunders (and are you sure about that?) but if you want them to keep coming around you need to give them more to do.
Depends on the newb of course.
I didn’t think of the psychological aspects, but it won’t take more than 1-2 rounds until the US lands in Africa, and from there the US player will see some conflict, or at least get closer and closer to some important dice rolling.