Why does it have to be a who? It might be a better idea to have natural features like the Appalachian mountains, or man-made structures like the Statue of Liberty, or perhaps American inventions or achievements like the Moon landing--i.e. instead of a person, a place or a thing.
If we do go with people, it should probably be RL ones, as Atlasians would be awkward given that several contenders are still active in the game, and there would be inconsistencies and points of contention over what pictures we use for them. Should we go the real life people route, some nominees could include (only listing dead people, partially to keep me from just hackishly naming a bunch of people I like): MLK, Thomas Paine, Susan B. Anthony, Mark Twain, H.D. Thoreau, Muhammad Ali, Jackie Robinson, Jimi Hendrix, Charlie Chaplin--or if those suggestions sound too fun and interesting, go the boring route and pick a bunch of old Presidents.