Because there are literally hundreds of choices of optics to use on an AR-15, it's ok if you're confused on which one you should get. The truth is that there isn't one perfect option. The trick is to pick the best one for your specific use or application. There are three main types of AR-15 optics which are named after their respective uses: Close-Range, Mid-Range, and Long-Range.
Close-range optics are generally non-magnified red dot sights. They are designed for use at point blank range out to approximately 200 yards. They can be effectively used at longer distances, but usually require a significant amount of hold-over (Kentucky yardage) because of bullet drop. These optics are usually reduced-parallax or parallax-free which allows both eyes to be open while shooting, greatly increasing your field of view. Red dot sights are usually the most appropriate optics choice for defensive rifle use and most anything within 100 yards. Some great choices for close-range optics include the Burris XTS-135 Xtreme Tactical SpeedDot, EOTech 512, and the UTG SWAT Force Red Dot. In order to extend the effective range of your red dot sight, you can add a magnifier like the Mako 5x red dot magnifiers.
Mid-range optics are typically low-power magnified sights that feature aiming points for distant targets. They are optimized for 50-400 yards. Mid-range sights are typically 3 or 4 power fixed magnification or are variable power in the 1x-4x range. Despite the fact that these sights are not designed specifically for close-range use, they usually work well for close targets if they have a brightly illuminated reticle. The main goal of a variable-power mid-range optic is to provide target acquisition speed similar to a red dot, with the distance capabilities of a mid-range scope. We have several great choices for mid-range optics, including the UTG Accushot CQB, the Leupold FX-II 6x36mm, the UTG 5th Generation CQB Scope, the Millet DMS-1, and the Leupold Mark 4 CQ/T.
Long-range optics have the capability to engage a small target at extended ranges. These scopes are generally variable magnification with a minimum power of 3x to 4x and a maximum power of 9x to 16x. Their reticles are usually some type of mil-dot or mil-hash with very thin lines. The long-range scope is designed for extreme accuracy at long distances yet, when dialed down in magnification, remains useful for mid-range distances as well. If a long-range scope is right for you, take a look at the UTG Accushot SWAT Scope, the Leupold Mark 4 LR/T, the Millet TRS-1, and the Burris Fullfield II.