My money would be on Kim Jong-un because he's a complete headcase. Makes Trump look sane.
I don't think he's going to launch a nuke because I don't think he has any just yet, but I reckon he'll launch a big fuck off missile at Guam during the next few months.
Then Trump, South Korea, Japan and Australia will bomb seven shades of shite out of him. First they'll take out his ground to air systems and then they'll drop one or two of those MOABs on his house. Game over.
Trump and his allies know that they can't strike first because then the Ruskies and the Chinese will get involved. And they can't bomb his nuclear testing site for the same reason - bombing that will release radioactive fallout.
Tbh all of this should have been addressed YEARS ago but Obama bottled it.