Finally what about Deep Space Objects?
These have low surface brightness. Since even the sky has some natural brightness, it is critical to get as far away from the lights as you can. That needs to be your first decision on site selection. Since the goal is usually to just detect an object then Transparency become the key to success.
Beyond that get as high and dry as you can. The weather links contain some information on the current weather at each site. I will try to supplement this in the next few months with some climate data. In the mean time refer to the TAC sites page for the locations and quality of particular sites.
Seeing is not as critical except for the large bright objects. The references give an example of M74 to illustrate this point.