Difficult to say. 10 years ago, I'd have said the US without a second thought. Now I'm not so sure.Depends what kind of war - I mean if it was trade saanctions first of all and all that kind of stuff, the US would take it due to supply routes, raw materials etc and the revenue generated there.
Then after sanctions it turned to all out killing machines, and assuming someone dont 'hit the button', I'd struggle again. China has become a killer economy of recent, and its manufacturing capailibty, offensive or otherwise cannot be disregarded.

Still undecided, but do bear in mind it was one of them that wrote the rule book.
Rule No. 1 - Know your enemy.