The Rest is History did a series on Kennedy's assassination last year. It was very good.
In the last episode, Sandbrook went over the theories and laid out two really important facts that I found persuasive.
One, Oswald was mentally ill, troubled as a child, and very erratic as an adult. The notion that the CIA or any other intelligence agency worth its salt would count on him to be the assassin is unlikely, to say the least. That's just not how those services operate.
Two, the very shot that everyone said was so difficult that it couldn't be done by a normal shooter with that rifle has been recreated using the same make of rifle.
As to your first point, most of the conspiracy theories don’t have Oswald as the assassin. They have him as the scapegoat. He was recruited
because he was the way he was.
As to your second, as he wasn’t the assassin, they weren’t relying on him making that shot (assuming he was even the gunman on the 6th floor…many question that). In fact, virtually all of them have the fatal shot coming from somewhere in front of the limo.
That said, in the Marines, Oswald took the marksmanship test twice. First time, in 1956, he got a good score. Second time, in 1959, he got got a bare minimum score.
And, again, it wasn’t one shot. The WC put it at 3 - in roughly 5 seconds, based on Zapruder analyses.
Shot 1 they claim missed. And everybody agrees that one shot did miss, whether it was the 1st or 2nd shot fired. Shrapnel actually hit somebody.
Shot 2 hit POTUS in the upper back and continued on to go through Connally twice. This is very doubtful, for numerous reasons. Connally himself maintained to his death that he wasn’t hit by the first shot. And the Zapruder film casts doubt on this too. JFK is reacting for sometime before Connally is.
Moreover, this is the actual bullet the WC said went through human flesh and/or bones
3 times before lodging in Connally’s leg.
How could it still look like that?
And then there was the fatal headshot - which was either the 3rd or 4th shot fired (depending on who you believe).