I’s a controversial position, but I think ‘Jackie did it’. Frame analysis of the Zapruder film shows the killshot trajectory emanating from her lap or the seat. i think the spatter evidence on the pink suit will someday verify it. There probably was a bushy knoll shooter, and the real Lee Oswald did not participate.
Why Jackie? They were estranged by that point. Her background, family connections, and the month with Onassis offer many possible motives.