User blog comment:ImperiexSeed/Supernatural Mysteries/@comment-1509826-20120312174351/@comment-4920092-20120312180431

A lot of users seem to theorize that a horseman is as powerful as his particular attribute (this idea comes from the episode where Famine is frail, old man because of the overabundance in the modern world). That said, everyone dies. In fact, tons of people are dying right now. And death has ALWAYS existed. Famine and Pestilence cannot exist independent of death (that is, if they exist, so does death, as they can cause death), but Death can exist without either famine or pestilence, for everything will die, even without the aid of disease or starvation. And war, war didn't exist until humans had consciousness (which, according to the show, wasn't until the angels gave it to them).

Thus, it makes sense that Death is the most powerful. Death has always existed, and will always exist (so long as there is life).