JFK.
1. He believed in what he was saying when you look at him giving speech
2. He has got some sort of idealism which was validated with him being killed by non foreign agent. If it had been foreign agent WWIII would've already erupted or at the very least, a counter assassination of the Soviet highest leader would have happened. It's that obvious.
3. His speech about an evil secret society, coming from a US president in office no less
The greatest modern day American however in my opinion would be Martin Luther King Jr.