Kennedy, far greater president than Goldwater would have been.
I really can't think of anything that Kennedy did that was great. He might have turned out to be great with a full 8 years, but I don't think the chances of that are that great.
He gave tepid support to the civil rights movement beginning in 1963, but who knows if the civil rights laws would have past had he remained president. Johnson had considerable legislative skills, and sold the laws on the basis that they were a Kennedy legacy, to a large degree.
Vietnam was a mess waiting to happen, and Kennedy compounded it by participating in the overthrow (and resultant assassination) of South Vietnamese President Diem. His liberal apologists claim that Kennedy would have pulled out of South Vietnam as soon as he was safely re-elected, but I don't find that very convincing. It's more of a psychological fig-leaf to reconcile their hatred of the Vietnam War with their irrational devotion to Kennedy.
As I've said before, JFK is my favorite Kennedy, but I still don't think he was, or would have been, a great president.