Crikey - how long is a piece of string?
What will you be using it for? If hunting, what sort of distance are you expecting to shoot over and what magnification are you comfortable with?
The mainstay of air rifle scopes has for year been the 3-9 X 40 scope which will do the job pretty well. If you want to upgrade from that you can get bigger magnification ranges and objective lenses, that allow more light in. That being said, I believe that most shots set a magnification and leave it at that. On my rimfire I actually have a fixed 6 X 56 scope which I am perfectly happy with, but my air