Galaxies
I like galaxies. Images of these objects are awe-inspiring and stimulate the imagination. Just think: here we are tucked away in one tiny corner of a typical galaxy among billions. The star clusters and nebulae within our own galaxy inspire such wonder as it is. Imagine the life that may exist elsewhere in our galaxy. Now imagine our Milky Way, replicated billions of times over, and more extreme in some cases!
I am challenged by galaxies. Experience has shown that they are very tough to process properly. With few exceptions, they are also very tough to image properly! One needs a long-focal length telescope (at least 2,000mm) to capture an image of one that reveals impressive detail.