What is our biggest threat to national security?

The Islamic State of Iraq and Sham (better known as ISIS).

I think that all of these (and more) pose a risk...however, I think that North Korea poses the greatest risk as their leadership appears to have no filters or restraints. Russia is an old hand with "cold war" experience. Iran...and China both pose a threat, but when it comes down to it, will show restraint and come to the table to talk. N. Korea? Think they are a loose cannon....

Russia, China and North Korea. All for different reasons.