I have a love affair with Armalite, very nice quality rifle with good fit and finish, pretty much astounding customer service(in my experience), and ofcourse they've got that lifetime warranty on their Armalite rifles(Eagles are a different warranty, more value based intent). Bout the only drawbacks I see with Armalites are the price of the rifle, a slightly limited selection of rifles, and if you're plum crazy to get a rifle as close to mil-spec as possible than Armalite doesn't match Bushmaster in that respect.
I also like Bushmaster quite a bit, they are a darned decent value for what you're getting. As close to mil-spec as you're gonna get in an civilian AR15 without it being an M16. Price is certainly pretty fair on the Bushmasters when you consider you're paying the price on quality like which can happen with say ASA or Hesse, hopefully everyone has learned to shy away from ASA and Hesse by now! Bushmaster also has a fine selection of rifles all based around the chrome lined barrels and most recently they've come out with their Varminter with a more match type barrel geared more for accuracy.
While I don't own one I've heard very good things about Rock River Arms, I believe the reports I've heard about them and take it for what it's worth, they're good stuff and likely as good a value as Bushmaster.
Those are my top 3 choices for AR15s if you're in the market for a pretty basic AR15 and nothing too out of the ordinary.