@p the story goes that when the shooting from the book depository started, the motorcade sped up suddenly, and a guy who was standing up in a car behind JFK fell down and accidentally fired.

I don't place too much weight on it, but I'd say it's my favorite because it's goofy. if you put a gun to my head I'd say CIA, steel industry, mafia or mossad because each of those has a clear motivation. it could be a mix of both too, because I believe that oswald was a CIA asset, and the SS theory depends on him shooting from the building.