Which tool is most commonly associated with nation state threat actors?

Advanced Persistent Threat (APT)

An advanced persistent threat (APT) is a set of private and perpetual computer hacking methods, often composed by a person targeting a definite entity. An APT ordinarily targets either individual organizations, states or both for marketing or administrative motives. APT methods need a high quality of covertness over a long period of time. This "advanced" system implies advanced methods using malware to employ vulnerabilities in systems.