While I consider the topic of this thread a bit odd its also interesting. To answer the question, I'd say the US hands down, its just no contest. No single army in the world would be capable of defeating the US armed forces as far as traditional warfare is concerned. Its not the sheer size of the US forces (they certainly are pretty big though), but the level of technological superiority, funding and training combined. Like MrsSkywalker pointed out its also that unlike other countries the US got all their bases covered, there are next to no weaknesses. Some interesting figures: the US military budget is higher than Russia's total DGP; the US military budget is as large as those of the next 20 nations combined and over 6 times as high as the second largest (Russia).
China has numbers but not (yet?) the equipment and technology, Japan has smaller forces but more equipment (esp. air and naval forces) than China AFAIK. Russia still has a lot of know how and what can probably still be called the world's second largest war machine, but even though the Russian government spends about 20% of its DGP (!!!) on the remains of the red army, it still is in slow but steady decay and dire need of modernization. The UK and France probably come closest to the US in regards to technology and training, but the sheer numbers and hardware isn't there. Germany is pretty much a sitting duck, the level of technology in some areas is good and the special forces are top notch, but overall training and mobility are rather lacking.
Huge standing military forces today are pretty much a leftover from past times IMO. And while the global military spending has gone down as much as 50% over the past decade and will likely stay down there (except for a few possible exceptions in Asia), the US' is rising again and plans are to continue this trend. The threats of the coming century are hardly best met with traditional large military bodies (or expensive big-ass cold-war-relic weapon platforms like the pentagon has in mind), but rather small, highly specialized, well equiped and trained special forces as well as increased intelligence efforts. I think military bodies worldwide need to undergo significant structural changes to adress the threats of this century. The traditional armed forces are not obsolete, but should be downsized in favour of such smaller specialized forces which would also have a much higher readiness level.