Well I would've said science or philosophy since the ideas that arise from these endeavors are timeless as opposed to political and military actions that can often be transient. However, the more I think about it, the more I think that it is not the case. Most scientific discoveries would have occurred at about the same time without the individual who is ultimately given credit for the discovery. Examples are evolution, Darwin and Wallace; the calculus Newton and Liebnetz. It is even more the case today, as there are many more scientists, that most new ideas are arrived at at about the same time by multiple individuals. Thus, the effect of the individual, even though they may get tremendous credit and prizes for being first, is not that great. On the other hand political decisions made by an individual can sometimes be quite critical and lead to very different results. Imagine for instance that someone like George Bush was president during the Cuban missile crisis. Would any of us be here today? Would he have opposed the advice of the majority of his political and military advisers and not invaded or bombed Cuba, a course which we know now would have likely lead to a nuclear war?
EDIT: By impact on humanity I do not mean just in your lifetime but your impact summed over the course of all human existance (however long that may be).
EDIT: By impact on humanity I do not mean just in your lifetime but your impact summed over the course of all human existance (however long that may be).