You can build or have one built that is much better than most name brands. With that said though, I'd get an LVOA from Warsport if money was no object. They are bad mofo's.
For the money, Rock River makes a pretty good AR. Their factory trigger is nice. Most factory triggers are crap. I bought a custom AR in 7.62 from a guy in Florida in December and dropped in a single stage 3.5lb pull CMC trigger. Makes the world of difference on accuracy. I'm sub MOA with crappy ammo with that trigger and barrel set up. Another decent option is DPMS. They're made by Remington.
You might consider getting one chambered in 6.8spc. Little more power than a 5.56 and bullets are readily available now.
As far as optics go, I highly recommend Vortex. Lifetime no questions asked warranty. I have a Vortex Viper HS 2.5x10 on my 7.62 and a Crossfire 3x9x40 w/ illuminated reticle on my bolt .223. The crossfire is their low end model, but you wouldn't know it by looking through it.
www.cameralandny.com has great deals on optics, Vortex or not. On my next build I'll be putting on their new 1x6 Strike Eagle, probably on a .223 or 6.8. Another thing to consider for optics is what you're going to be using it for. Plinking, hunting, target shooting, etc. The point above about putting high dollar scopes on AR's being useless does have some merit, but I use my 7.62 for hunting, so I paid more to have a scope that is clear and very good in low light situations.
You'll want dtspoke to chime in on this subject. He seems to be the resident gun expert.
