It all depends on what type of gamer you are. If you like FPS games, I'd recommend Arma III, Battlefield, Half-Life 2, Modded Skyrim, Counter Strike: Global Offensive, and one of my personal favorite underrated games - Insurgency.
Many PC games that every PC gamer should play are made exclusively for the PC and the reason is actually valid. Minecraft w/shaders is a given. Guild Wars 2 is similar to World of Warcraft but in my opinion it has a completely different feel. If you like RTS games, i'd recommend the Total War series, Civ 5, or the Company of Heroes 2 series.