Historically, the "big" newbie nation has been Italy, and I see no reason why that would change this time around. You've got easy early wars, a flexible warmachine, and if all else fails Germany will be on your doorstep, ready to make up for any shortcomings on your end.
And let's face it, it would take a very special kind of "talent" to do worse than Moussolini did
Runner-ups in the past have been... the rest of the axis - Japan and Germany are both flexible countries, with relatively easy first conflicts and a plethora of tools to play around with.
As mentioned by others, the changes to the Munroe Doctrine should also make for some interesting games in south america - the top-dogs in Brazil and Argentina should both have enough tools to roll over the rest of the continent, ending with a climacting final showdown against their regional rival. Time will tell how it works out in practice, but it at least seems solid on paper....
I've also been thinking the Raj might be an interesting early option - you've got decent resources, early entry into the war, and enough distance from the front-lines to allow you to focus on building neato masterplans, without having to worry about screwing up your defenses. Kinda like a miniature US, 'cept with action prior to the end-game.
