To quote Beric Dondarrian from Season 7's Beyond The Wall, while talking about the Night King: "Kill him. He turned them all."
To quote Ralph Fiennes in Hail, Caesar!: "Would that it were so simple."
The Night King has emerged as the true Big Bad of Game of Thrones, and while he may not be as interesting a character as someone like Cersei, he's the most formidable foe our heroes have ever faced. He's the first enemy and the last, which is doubly true since the show is ending next year and the climax will no doubt boil down to having to defeat him. But that's easier said than done.
We've seen White Walkers killed before, but the Night King would appear to have capabilities beyond them. He seems to possess a fair amount of magic, we don't know if he has the same vulnerabilities as other Others, he rides an undead dragon, and he's an Olympic level javelin thrower.
It won't be easy for anyone to take him down, and even many of Westeros' greatest warriors would fall short. There are a few hardy souls, however, who may just be up to the task of dethroning the Night King and saving the day.