I would have to say the Hubble Deep Field photograph which was released in 1996, and showed thousands of distant galaxies in a region of the sky a few arcminutes on a side. Recent analysis of the distances to the galaxies shows few of the normal, well-formed spirals, and that the number of primitive galaxy fragments increases to a maximum at a redshift near 3.5, and then drops sharply at redshifts higher than 4.0. This means we are finally beginning to see, in detail, the epoch of galaxy formation revealed by the accumulation of these billion-solar-mass fragments into larger galactic structures at lower redshifts. The detailed study of this field suggests that most galaxies may have formed as recently as a redshift of 3.5 which is a time when the universe was 4.5-times smaller than it is today, had 91 times less volume, and had an age of only a few billion years. This places very hard observational constraints on even the most favored 'Cold Dark Matter' version of Big Bang cosmology, which would have preferred a more recent galaxy formation epoch.
The shakeout from all of this might be that if 'CDM' cosmology is not able to explain such early galaxy formation processes, then the idea it is based on, that the universe is exactly 'critical' as demanded by Inflationary Big Bang theory, may be untenable since no large quantities of dark matter will be needed beyond what we find in some galaxy haloes already.