I say 1984.
Which is almost poetic.
last fair election....
Only have to say 1956 because it is possible (not proven) the crucial majority gained by the Illinois electoral votes to allow the JFK win over Nixonin 1960 was in part due to thousands of City of Chicago votes for the GOP which went into the river in sealed boxes. Not that Mayor Daley was crooked. /S
Asked many years later about this, Nixon said “I have no regrets even if that is true. I might have been the one murdered had I won.”
What a country.
At this point it wouldn’t surprise me if 1984 was the last honest and fair election in the USA.