Which country among the is biggest threat to the world.. Russia, US, China, N Korea, Iran, Pakistan

A lot of people are going to say the US of course, and it's true that the US has the most intervention power. But in the last few decades the US has used that power the world hasn't ended. Superpowers actually like ruling the world, not destroying it. It's easy to emphasize this and that, and yeah, intervention doesn't always go according to plan, but I'm not sure the US actually poses the biggest threat.

Russia also has a highly interventionist policy, but are only able to carry it out in their immediate neighborhood for the most part.

China also has shown no qualms with expansionism through various means, though they do display a modus operandi of using both military pressure and soft power to do it.

North Korea is rogue, but Kim is also realistic and doesn't want to get killed.

Iran is hyped up a lot, but personally I think the Saudis are more dangerous than they are.

Pakistan...you know it might be Pakistan. Not because the government of Pakistan is powerful or has any nefarious plans, but because the nation is such a hotbed of out of control social and political f*ckery that I could easily imagine terrorists, insurgents, and rogue government officials in that place colluding to really mess up a lot of stuff. And Pakistan is nuclear armed! XD