I'm trying to decide between MLK and Abe. Both were leaders under a time of great duress in our nation - both continuing battles related to civil rights and both coming from poor backgrounds. In some sense, Abe's battle wasn't just about civil liberties for all - a lot had to do with states' rights and the emancipation proclamation was partially about rallying the blacks to fight for the union - IMHO. Both may have had greater impact with their deaths - impossible to tell since they didn't get to lead the rest of the way.
Reagan may have helped the downfall of communism, but I think it would have fallen eventually. If anything, it may have happened slower, which may have actually ended up easing the break up of the USSR.
As someone else mentioned, George Washington was one of several founding fathers and just happened to become our 1st president. If I recall correctly, other presidents also signed the Declaration of Independence and served in a governing position.
Ben Franklin was also a great man and of importance in history. I just don't feel that he was as strong an image of leadership as the others.