The most mind blowing started in 1995 when they pointed Hubble at what was an almost 'empty' patch of sky for ~140 hours over 10 days:
and found ~3000 galaxies. (click twice to zoom in)
Then in 2004, another slightly smaller patch, with new cameras, ~280 hours over 2 months, found 10,000 new galaxies:
Those two were northern hemisphere, so they did a southern one and found similar numbers. Thousands of Galaxies everywhere they look...
Video Hubble Deep Field The Most Important Image Ever Taken