I'm suprised there haven't been alot already.
You'll hear alot of opinions, usually this is what you'll hear:
1) Armalite - Good quality, good customer service
2) Bushmaster - Good quality, good customer servie.
3) Colt - Good resale value, various comments regarding quailty and customer service.
4) DPMS - Have heard some good things about them, A friend has one of their DCM type uppers and it was pretty good.
Now, MY PERSONAL opinion is go with the Bushmaster, Then Armalite or DPMS.
I have several Bushmasters and have had NO problems with them whatsoever other than the typical "poor quality magazine" caused a jam...
There are some other good ones like Sierra Precision, Les Baer, Wilson Combat, etc it just depends upon what you're looking for and what you are willing to spend.