Best bang for the buck, among your choices, S&W.
The now-discontinued (company is out of business) Charles Daly Defense D-M4LE was an outstanding deal, nearly the same features as a Colt 6920 for a lot less money. Those might still be available some places, but there would be no warranty.
The Colt 6920 is pricey but you would likely never find anything to upgrade. Similarly options from Daniel Defense, LMT and Bravo Company. With some shopping around you can find those in the $1200 range. Here's a Daniel Defense for $1199:
http://www.aimsurplus.com/product.aspx?item=F1DDXV&groupid=11
And that one even has the hammer-forged barrel, which is not mil-spec but actually better than mil-spec.
Other options that might be good values would be Armalite, Stag, and Sabre Defence.
DoubleStar and Del-Ton are good quality plinkers but if you plan hard use you might look for something nicer. I do not see a good value proposition for DPMS, Rock River or Olympic. CMMG might be worth a look too but is in the middle, good quality but not quite top end.
Shrubmaster/Bushhamster makes a mid-range gun that usually sells at an upper mid-range price, and some of them have bugs here and there (like undersized chambers), and they are near the last in cosmetics. I would take one at a great price, but not at typical asking prices. And I own two of them, FWIW.