What is the best Red Dot sight for an AR-15?

Aimpoint Comp M-4.

I think it is tough to argue anything is as proven as an aimpoint. The new MRO might end up being great or it might end up being like the SRS; time will tell. Personally I hope it is awesome and is the first legit competitor to an aimpoint micro. Between the aimpoint choices my preference is for a micro. The PRO is a solid choice if budget is a bigger concern. The H1/H2 will probably serve someone just as well as the T1/T2 if the person doesn't use NVGs and isn't going to scuba dive with it.

In sum if budget isn't an issue I'd get an aimpoint micro. If budget were a concern I'd shop for a deal on an aimpoint PRO.

I own a CompM4s, and a T-1 Micro and a PRO. They reside on my work/duty guns.

For your stated requirements I would go with an H-1 Micro or a PRO.

Late to the party here, but another vote for a PRO and a case of ammo with a $700 budget to equip an AR. Do you have any experience shooting with a red dot? I came to using a red dot from years of shooting with aperture sights and found it took a few hundred rounds to get used to it. The case of ammo is obviously to help with this.