Watergate. That led to the election of Carter. After that it was all down hill...including GWB winning....which led to Obama.
Watergate would be my choice also.
Undo the JFK assassination, no Saint Kennedy. Certainly no LBJ presidency, maybe no RFK candidacy or Ted in the Senate as some sort of sacred cow (over and done in by Mary Jo’s manslaughter).
Vietnam takes a different turn.
Goldwater and Reagan in 60s place in politics may have gone differently. Or even Nixon’s.