AR advice

Rock River is my vote. I'm a bit biased though see below.

What he said

Depends on price range. Colt, rra, Daniel defense, Bcm(bravo company). These are great weapons that won't exactly break the bank. Average cost is 800-1500 depending upon which one and condition. I am partial to Daniel defense personally, but you really can't go wrong with any. What are you primarily using it for? Factor ammo in as well when deciding which caliber. There are thousands of different routes you can go as far as rails, handguards, barrels, etc. Go handle a bunch of them and then make your choice accordingly. My suggestion to you is to go for chrome lined guts and barrel.

Use the search. There were about 3 AR15 threads in the past week.

All of them will run just fine. It's really about what level of accuracy you want, and how much you want to accessorize. I have a DPMS 20" upper on a RRA lower and it shoots amazing groups. But then again, it's heavy as sin and a 16" would be better for moving around.

Decide what purpose you want it to fill, and then you'll have a better idea.

do a build up, i really wish i would've with my first AR

Rock River would be my first choice followed closely by Stag.
Great rifles at a good price.