Which has repeatingly been stated in the past, it is impossible to really tell the powers of the Lich King. We know he has highly advanced powers over the spiritual realm, over necromancy, shadow and cold magic as well as a high extent of telepathic abilities. His intellect is most certainly high tier, which makes his not quite matching abilities just as dangerous as if they were. While he is of little significance in power to larger forces in the universe, such as some of the aspects, the Old Gods or the Pantheon, he is certainly far beyond anything that has an active status on Azeroth to date. Given Nozdormu's absence, Neltharion's departure and Malygos corruption, he really is in any man-to-man combat encounter unstoppable. Save for possibly Alexstrasza who would oppose a threat in an encounter (But as we all know, she would not engage Lich King in battle)
I base this on various Lich King official phrasings by blizzard, on his known abilities so far as well as various historical encounters. If warcraft characters were rated in matter of how dangerous they are on a destructive scale, I would personally say Lich King is perhaps 8/10. There is a big leap between 8, 9 and 10 though. This given his current advesaries, his abilities and superior mind. Had he risen in the middle of War of the Ancients, he would be further down on the scale because there would be factors that could oppose him.
As it is right now, given his advesaries, he is the most dangerous entity on Azeroth, alongside the corrupted Malygos (Whom fortunately seem to be sitting the storm out, only to be properly dealt with by Alexstrasza's and the horde/alliance hands)