If you are thinking in line of a Tactical type optic and not a scope.
The ACOG sight is a fine sight but also has some factors to consider. That being it has magnification so it makes any cowitness with the iron sights impossible. One must plan on using it exclusively. My recommendation would be a Aimpoint Pro or one of the EOTech Models with the 123 battery. Both have cowitness capabilities. If price is an issue you can go with an EOTech Model 512 or similar optic which uses AA Batteries. For them I would recommend Lithium AA Batteries or Industrial grade AAs.
The Constitution is not an instrument for the Government to restrain the people. It is an instrument for the people to restrain the Government!
Today, we need a nation of Minutemen, citizens who are not only prepared to take arms, but citizens who regard the preservation of freedom as the basic purpose of their daily life and who are willing to consciously work and sacrifice for that freedom."
-- John F. Kennedy