Honestly, I think Oswald did it. I used to buy in to the conspiracy theory until I moved to Dallas. I live, as a matter of fact, walking distance to the site Kennedy was shot and so I've seen it many times - seen the grassy knoll, the book depository, etc.
When you watch Oliver Stone's movie "JFK," the way the camera work is done it really looks like that window in the book depository high up and a long way off, ie it looks like it would take a real marksman to pull off the shooting. When you're actually there in person, you see that the location is perfect for what it was, that Oswald easily could have gotten off 3 accurate shots from there. The building is remarkably close. It would have been a turkey shoot.
What's crazy to me is that they allowed the motorcade to go through such a tight bend, that they allowed Kennedy to be in an open top convertible, and that they didn't block off nearby buildings like the book depository. I just have to chalk it up to being a more naive period in history, that a president hadn't been murdered since Lincoln, and to really shoddy work by the secret service on that day.
Stranger things have happened.