If war with North Korea comes, what are the U.S. military's most powerful weapons?

The most powerful? Soldiers, sailors, airmen, Marines, Coast Guardsmen, and the Reserves and Guard! Weapons are tools and our men and women are the best at employing them...

MRE's. Drop them on the enemy and watch them surrender. Seriously, we have a lot of various weapon systems that are very powerful and will do the job nicely.

Our most powerful weapon will the diplomatic capabilities we are able to employ that keep Russia and China out of the conflict, or even better see them cooperate with US forces.

When it comes to doing direct damage to North Korea, I'm thinking South Korea.
https://www.globalfirepower.com/country-military-strength-detail.asp?country_id=South-Korea

As for US military weapons, I'm guessing THAAD. Assuming we don't attempt a first strike, North Korea would be foolish not to unleash any and all of its nuclear capabilities at the onset of the war. THAAD and other missile defense systems will be the most important weapons we bring to the table.