I think Oswald was the one and only gunman. It's possible he had people behind him who wanted Kennedy dead, and hired him to do it or used him as a scapegoat, but most of the consipiracy theories have absolutely no evidence.

The most likely scenario is that Oswald was a frickin' Communist that hated JFK and wanted him dead... and made it happen.

The video footage clearly shows the exit wound was in the front of Kennedy's head, which means the killing shot came from behind--end of story.