JFK is my favorite.
Think about it for a moment - Who had the most weapons that morning in Texas?
Oswald took a pot shot, missed, and then the Secret Service all pulled their weapons. Secret Service agent on the car behind JFK fires multiple shots as he pulls out his weapon. It was an AR15. Think about it - it makes perfect sense. JFK was obviously shot from behind - Not from the Texas School Book Depository, not from the grassy knoll.... It's the only theory that makes sense.
Accidental assassin: JFK theory alleges Secret Service agent fumbled gun - NBC News