I agree, China is more of an economic/political threat than military one, unless the USA seriously pisses them off somehow or declares war on China, e.g. for attacking US allies in the region. However, the USA is China's biggest export market as far as I know, and it will remain so for some time. China is growing thanks to this, and they'd be crazy to risk an economic embargo or any war with their biggest trade partner. The US would respond at least on politcal and economical level when China decides to invade Taiwan, so China probably won't do that until they don't need the US anymore and get imperialistic. But of course, that may not take long or a lot...
Iran is a threat to the region... Israel + oil... then terrorists, so it's the biggest threat to US interests... especially with this nuclear program. Not that I think a resulting nuclear weapon would be a threat to the USA itself, but the results of a US(-led) or Israeli pre-emptive strike...
But if the US/Russia relation turns sour and cold again, Russia might become the biggest 'visible' military threat to US/NATO again, while China meanwhile takes over the role of world superpower.
I wish I could see into the future though, haha.