Bushmaster M4, painted with an ACOG
Having the greatest rifle in the world is pointless if you can't aim at what you want to hit. Kentucky windage is cool and all, but even the cheapest modern rifle is accurate enough to require some kind of sight (iron or otherwise) to use well. Not having a good sight means you are losing out on the potential performance of the rifle. So you need something to aim with. Depending on the role you want from your rifle, its caliber, the accuracy of the ammo you use and many other factors (including even how good your vision is and the availability of batteries) all make selecting a sight a very individual decision, but there are some optics that just make sense to pair with some calibers.
For people with a typical M4/AR-15 set up that do most of their shooting under 300m, a 1-4x magnification scope is a good pairing to consider. The 1x allows for extremely fast target acquisition with both eyes open and the 4x is enough for most people to maximize the accuracy of the round at ranges greater than 100m. I am in the process of reviewing some of the better options in different price ranges for a 1-4x scope and I will post the pictures, preliminary reviews and the final reviews as I complete them, but for me testing an optic takes more than a day at the range.
There are many, many options in both iron sights and reflex sights. I have covered some info on choosing irons on my other page.
However for a zero magnification illuminated optic, there are 2 household names - Aimpoint and Eotech (review here). Of the 2, I personally prefer reticle of the Eotech, but overall I much prefer the Aimpoint. There are some up and comers though - you can't set the MSRP as high as Aimpoint has done and not expect some people to copy you. They left the sub $500 red dot market wide open and brands like Vortex and Nikon are doing their best to fill it. I have used the bottom level brands like Truglo, UTD and Bushnell and while they do work, they are not constructed to the strength or quality of the other brands I mentioned. Depending on your rifle use, they may be all you need, however my site user base is mostly guys that want to run a a gun hard or get the most out of a gun. If that is you, avoid the cheap lines of red dots.