What is the most important leadership core value?

Great question and it's important to break that question down: What we "value" is what we place importance on or what we find worth or significance in. Central to leadership is the ability to influence so that followers can be gained. Even the leaders that have no significant title, passion, or integrity may become "great" ("great" being defined as exceptional in degree or intensity). Hitler had limited integrity, but could be defined as a great leader because he influenced many followers. Ghandi did not show his passion and had no formal title, but he influenced many followers. There are even some leaders that don't even want to be a leader, but people follow them (a scene from Forrest Gump comes to mind....fictitious but could be true). All of these leaders have one thing in common: They're going somewhere. So...from an outsiders' perspective, the outsider would value the ability to influence as the most important and from an insider's perspective, the leader would value where they are going (vision) as most important.