I really wouldn't recommend that someone who has to ask "what brand should I buy?" build his first AR. At most, he could just buy a complete lower and a complete upper and then just slap them together.
The fact of the matter is that there are lots of very good ARs out there. You may argue, as I did, that one brand is a bit better than another, but odds are that you will never realize the differences. I have a parts AR that I put together using parts from good manufacturers. I used a Daniel Defense upper, Bushmaster lower, Bushmaster barrel, BCM bolt carrier group, and other odd parts that I got from BCM. I don't even clean that rifle unless I need to inspect parts for wear and it just keeps on running.
There are just a few things that I would look for that give me piece of mind and insure compatibility. First, I like to go with a full auto bolt carrier. They are a bit heavier than their semi auto counterparts and will work in conjunction with things like a heavier buffer to smooth out the shooting experience. I prefer a mid length gas system on anything with a 16" barrel or a rifle length on anything longer than 16". I prefer a milspec buffer tube, as opposed to a commercial spec buffer tube. I like my bolt to be MP and HP tested, just for piece of mind. I also prefer a chrome lined barrel. It just plain makes the barrel easier to clean and maintain. If I remember, BCM barrels are also HP and MP tested for added piece of mind.