Armalite - Most likely to be the most expensive, but in this case you will get what you pay for. Best warranty. Top notch in every respect, and if not, they will make it so.
Rock River Arms - New kid on the block who is out to make a good name for themselves. When I buy another AR-15, these guys are going to be on a very short list of desirable manufactures.
Bushmaster - Have had a few issues with purple finishes and barrel nut torque, but they have made good on their word and fixed the issue for the customers who sent their rifles in for repair. In my opinion, the above issues should have been caught before the rifles left the factory. And in all honesty, it has been quite some time since I have heard of anyone reporting these issues. I believe they have fixed their minor QC issues and are producing a darn good rifle at this time.
The above 3 are exellent choices 99.9% of the time. Find the best price for the features you want and get it. You will not be disapointed.
Colt - Non standard parts and over priced. They do have a following among the collectors though, and among those who are stuck on a name, but they are not on my list for a shooter or a SHTF day dream because of the parts issue.