The Eagle Nebula has always been one of my favourites. It's the apparent size, texture, and the 3-dimensional aspect that does it for me. It also resembles the type of abstract images that I consider aesthetically pleasing to the eye.

I also like the image of M81 and M82. It says to me "Look! Here is one galaxy... And there! Off in the distance is another!" I like to stare at what appears to be thin, barely luminous gases defining the edge of the galaxy. I try to imagine what it really is: hundreds of millions of stars scattered like so much dust, each one an astronomically large ball of nuclear fusion, yet only visible at this distance because of its innumerable neighbors. Also consider that each star is a mutually isolated, potential biosphere, and that by looking at that photograph we are probably taking in countless civilizations, each one totally unaware of each other's existence.

The Hubble Deep Field photograph is really profound. I see hundreds, perhaps thousands of galaxies? That piece of sky is probably equivalent to that which is obscured by the head of a pin held at a distance of 5 miles. Whoa!