No Japan. Definitely not the five most powerful. Economics is what Japan has going for it....
EU? The European Union is definitely not fighting as a single army, so disregard that one.
I believe China has the largest air force in the world, as well as the largest army (1.7 million). U.S. is, I believe, actually 6th (475,000) in terms of pure size of its "army", behind (in order), China, India, NK, SK, and Pakistan. UK only has 105,000. That said, it's quality that matters, but it depends on what type of conflict you plan on fighting which would be strongest. Also, the advancement and power of air force and navy upset this numerical advantage in the armies. France, Germany are the other two European powers that deserve consideration.
Fighting on their turf, you wouldn't want to mess with China or India. In terms of their ability to bring the fight elsewhere (except for their immediate neighbors, esp. in the case of India - Pakistan and their ongoing disputes), they lack behind more technically advanced European powers.
So... (excepting nuclear weapons... they make everything else moot, basically, and assuming the fight was on neutral ground, therefore no local advantage, and also assuming this is symmetrical warfare)...
I'd guess:
1. US
2. UK
3. France
4. (tie) China
Germany
6. India
No Russia... how far we've come. BTW, I'm by no means an expert, just a a casual interest in military history and technology, so go ahead and correct me if I'm mistaken.