There are a Multitude of scopes on the market and dozens of manufacturers so the Question is How much do you want to spend, Do you prefer a certain reticle, Are you going to shoot in low light, Is it going to have to take abuse ?
These are all factors that have to be considered when choosing a scope.
I personally prefer Leupold, Redfield,Weaver, and High end Tasco scopes,each chosen for the rifle and the conditions it will be used for.
A 3-9 x 40mm is a good all around size/magnification for general hunting and target use on 30 cal. rifles.
Rich