President who died before before he could be impeached?

Nobody has been elected U. S. President and died before taking office.

In 1872, Horace Greeley died between Election Day and the day in December when the electoral college elects the President and Vice President, but based on the Election Day results, he would have come in second to Ulysses S. Grant anyway.

Also, there's a good chance Robert F. Kennedy would have beaten Richard Nixon in the Election of 1968 if he were not assassinated during the campaign.