Arguably the most important weapon on Game of Thrones right now is dragonglass, the common name for an obsidian-like mineral in Westeros. Along with Valyrian steel and fire, it makes up the holy trinity of White Walker-exterminating weapons. And it is very, very effective.
The first time we witness said effectiveness is on season three's "Second Sons," when Samwell Tarly uses it to protect Gilly's baby against a White Walker. Then on season six, Meera Reed, Bran Stark's companion — displaying the most awesome aim ever — uses a dragonglass-tipped spear to destroy a White Walker.
As the White Walker threat looms ever closer on the horizon, might we suggest stocking up on ye olde dragonglass? Yes, Valyrian steel is very fancy, and dragons — and fire — make for good PR, but dragonglass is tried and true and much easier to come by.