Astronomy itself is perhaps arguably my favorite since it brings me a new favorite almost every week; like a newly discovered flavor of ice cream that's even tastier than the last 26 I just had.
Nevertheless, just a few favored flavors are:
1) There is a nebula (Boomerang) that is colder than space.
2) No telescope, regardless of size, can make an object brighter (unless it's a point source). IOW, the surface brightness (mag. per unit area) won't increase more than your eye would see it.
3) That neutron stars (magnetars) can flash to be the brightest things in the sky, in terms of total energy, not visible light.
4) That all the crumb collection and our little brains could establish the Big Bang theory that fits so many of the observations and with no real competition, but...
5) We still don't understand the true essence of gravity; where's the gravitons?
5) That the Sun ain't even a tint of yeller (as seen from space). That was colorful surprise, pun intended. It slipped through the cracks due likely to an unusual set of circumstances, IMO, including the importance, justifiably, placed upon spectroscopy.