I've shot Colts, S&W and Bushmasters and they shot well but my buddies say they don't have the quality lately.
DPMS has a good reputation and I think they manufacture parts for many of the big names but I don't know about their present quality.
Make sure you get a 5.56 Nato chamber not .223 Remington.
To get the most out of the cartridge get a 20"+ barrel, if energy isn't that big a deal then you can go with the carbine lengths.
The trend seems to be toward piston driven as opposed to the gas system of the AR, cleaner and more reliable. There are conversions or you can just buy it as a piston driven system for a little more. I'd definitely go with that
Something to take a look at is this by Benelli, takes AR mags and is piston driven and very reasonable for Benelli quality. I haven't shot one though.
I really agree with the gas piston idea. AR accuracy AK reliability. The op should go here http://www.lwrci.com. I bought a different gun but if I had to do it over.
“The trend seems to be toward piston driven as opposed to the gas system of the AR, cleaner and more reliable.”
Bushmaster
http://www.bushmaster.com/products.asp?cat=6
5.56mm/.223 Caliber Gas Piston Upper / Bbl. Assembly
(About #26 down)
I have a bushmaster carbine. The last time I went to the range I was politely accused of cutting the barrel to an illegal length.
It also tends to be louder than the longer barrels which draws attention.