The "which Brand is best" question is a very controversial topic here.
Here's my take.
Excellent
Colt
Bushmaster
Wilson Combat
Barrett
Very Good
RRA
Armalite/Eagle Arms
Fulton Armory
DS ARms
Stag Arms
CMMG (now make their own brand)
Good
Olympic
Cavalry Arms
DPMS
DoubleStar
Fair to Poor
American Spirit ARms
Avoid at all Costs
Hesse
Vulcan Arms
Parts guns made of parts of unknown origin.
The new one to watch is Leitner-Wise, they have the potential to be the latest "new best thing" out there. Also note that rifles built on a stripped receiver from parts of varied sources can range from excellent to abysmal depending on who built it and what parts were used. Beware parts guns built on big name receivers being pushed as the real McCoy.
Unfortunately, the AR15 is so modular and so widely cloned that authenticity of used and to a lesser extent new rifles can be hard to determine unless you are familiar with markings and characteristics. Of course the modularity and wide availability of parts and accessories is also what makes them so appealing and addictive.
For your first rifle, as a newbie, so as not to get screwed, you are best off to get a factory complete rifle with new warranty.
Just my 0.02 cents.