Personally, I would say all the ultimate core books, so ultimate combat, magic & equipment. But if you are not so into the classes there, at least ultimate equipment because basically almost 80% of all the interesting gear is from that book.
I'd also say pathfinder unchained, if only for allowing unchained rogue, its a much better way to play rogues and means its likelier someone picks up playing a rogue if they are thinking about it.
