AR-15: EOTech optics
When it comes to optics, EOTech is one of the best know names in the industry. EOTech produces non-magnified "holographic" optics. Holographic is a red-dot optic that only shines one way. Meaning, the shooter can see the circle and red-dot (known as a reticule) when looking at a target. From the target side looking at the shooter, the reticule is not visible. Traditional red-dot sights will have a visible reticle from the target side and possibly giving away the shooters location.
This provides an obvious tactical advantage which is one reason it is widely used among law enforcement and military. The reticule is also adjustable for different lighting and works well in both dim and bright lights. Battery life is okay, but there are other alternatives on the market which outperform the EOtech in that department. As I mentioned before, the EOtech in non-magnified, which is also referred to as 1x magnification. I'm not sure why they don't just list it as 0x but whatever. Non-magnified optics are used when the shooter needs to quickly engage a target and keep both eyes open. Shooting with both eyes open allows the shooter to be more aware of his/her surroundings.
EOtech makes various models and some are night vision compatible meaning if the shooter is looking through night vision, the reticule will still be visible. However, NV-capable does add $100 to the price tag so don't buy it if you're never going to use it. Durability is another great thing about the EOtech products. They are tough and built to withstand various types of harsh treatments. Temperature, humidity, submersion, and impact are part of product development.
I personally dropped my rifle on an asphalt parking lot and my EOTech XPS took the entire impact. It left a nasty scratch on the hood of the optic, but it still works great.
Beware of knock-offs. There are some floating around online and most of them are still pretty expensive. I can guarantee they won't hold up when dropped like mine did. The glass used in the optics will show reflection unlike the genuine ones, and the buttons don't hold up either. I have compared mine to a fake while in a very humid indoor range, and fake started to show a very blurry reticule which was almost un-usable.
If your are going to spend the cash, get a real EOTech and make sure you get one from a reputable seller. The models shown below are guaranteed to be genuine and one even comes with the flip to side magnifier, a must for long distance shots.