I think a lot of it has to do with definition of important
if you define it as "made important discoveries" well those discoveries (science) are all bound to take place eventually, but we can argue for days about which scientist made the most important of the discoveries.
if you define it as "effected the most people" then which ever religion effects the most people.
jesus is the winner
then there is "who conquered the most". which would be hitler, Caesar, Alex the great (Aristotle's student for who ever was trying to make that list) those guys.
I guess it should be defined as "made the greatest lasting impact on human society" which is why this is such a good debate.
I think that these people only made the list because of american education. Columbus, luther king, washington. I mean they were all great people and what not, but in terms of the rest of the world they meant nothing.
so yeah, i think the answer should be subdivided, into categories, then the answer is a debate between which category is most "important"