If you are on a budget, there is a great red dot sight out there that I haven't heard anyone mention. It is the Millett SP-2. I recently bought one for my AR, and I love it. It has a 5 MOA red dot. The brightness is controlled by an 11 position rheostat switch. It has fully coated optics, is fog-proof, shock-proof, and most importantly is parallax free. It was easy to get sighted in, and has maintained it's zero very well.
I bought mine online from Impact Guns in Utah.
It was $79.00 plus shipping, and includes the 3 volt lithium battery, and weaver-style rings.
I wanted an Aimpoint ML2, but didn't want to
spend that much. I am not saying that the Millett SP-2 is just as good as the Aimpoint, but, IMHO the Millett SP-2 is a very nice sight
that is well made, and is very user-friendly. The matte black finish also matches my AR nicely. Just my $ .02