We've had a few discussions in the past about the JFK assassination, so I'm not going to dredge up all the theories again.
The Stone film was a mish-mosh, in which he incorporated parts of other theories along with Jim Garrison's. I certainly wouldn't rely on it as a factual representation of history.
The fact is though, that in my opinion there is a lot more credible evidence of a conspiracy of some sort than there is for the theory that Lee Harvey Oswald acted alone.
The problem is, after reading about 20 books on the subject, I'm just not sure which conspiracy theory I subscribe to.