Who is the greatest threat to the national security of the United States?

China is stable enough that I don't think it's them.

So who isn't stable... The list is huge. Who isn't stable and has ability to alter US security in a meaningful way? North Korea? Questionable. Russia? Also questionable. Which sort of leads us to nonstate actors. As much as people think it's played out, in my opinion our largest threat is still terrorism/extremism.

The United States. Especially our entitlement programs and debt.

Well ignoring the damage we're doing to ourselves, I'd say Russia, still, with their still-vast nuclear arsenal, and aged, yet still quite potent military hardware. In my opinion, a non-nuclear direct engagement with China would be over in a matter of days (this statement deliberately not including occupying China), simply because most of their hardware is inferior -- reference their new 'stealth' jet that can hardly turn, or their new 'aircraft carrier' that can't launch anything. They have a very few decent subs, and many more sub-par ones. They're much further from military parity with the US than Russia is, still.

Having said that, a Sino-Russian military alliance against the US would be...very bad. And some nut job lone wolf microbiologist...maybe worst of all.

This administration.....followed by self interest groups....congress.......2+ term politicians.....and the list goes on and on.

Iran and China for right now it will be Iran the most in the wisdom of our President he thinks that we should give them billions of dollars and they will be our friend that is one poor delusional man and if the Republican Party is not going to call him out on it I will aiding the enemy Iran is not will not ever be our friend well carry is making a deal with them they're out on the streets shouting death to America it's in our face are we that stupid our elected officials are obviously dare I say it yes I will treason let me make this perfectly clear TREASON.

Someone with Great Knowledge of Cyberwarfare but not too concerned about how the use of it will be viewed. I would say North Korea although I am very aware of the Information Warfare capabilities of ISIS also.

China both for the size of their military and for the money we owe them

Our own administration and legislators

Our failed education system is our deadliest adversary...

Insider threats. To many people getting involved with the wrong people.