First you have to establish what criteria to apply. I guess most people will agree that a person is important if he had a positive impact on the life of many others. Probably, saving lives or improving them considerable is a consensual criteria.
So, negative people, like conquerors, war mongers, and others, must be excluded. Even important people like Columbus, should be excluded, since its discoveries provoked millions of deaths on indigenous people between wars and diseases.
I prefer also to leave aside religious people, since they are controversial. Many had an important positive impact, but some were also cause of wars, executions, repressions, terrorism, etc. (I know, excluding religious people is very controversial also).
Also people of great intellectual impact must also be excluded if their thoughts didn't cause a direct practical improvement on the life of others (so forget about Einstein, Darwin, Plato, Aristotle, etc.; and even Newton or Galileo).
I also think artists didn't improve life enough (they shore help a lot, but...), so forget about Mozart, Beethoven, the Beatles or even Da Vinci.
My guy:
Alexander Fleming (he doesn't even is on the 100 Time list:
http://ideas.time.com/2013/12/10/whos-biggest-the-100-most-significant-figures-in-history/).
You don't have a clue what he did... Google is your friend.