I would nominate the 1937 census which finally saw the light of day around 1990 (though the first figures to surface seem to have been just the preliminary total). So many estimates had been flying around that the unpublished official return was desperately needed. The irony is that covering it up and leaving the inflated growth claims of the early 1930s uncorrected and the 1939 re-run highly suspect did more harm than releasing the disappointing numbers.