If I could only have one AR-15 in 5.56 I would choose the most relaible: Colt LE 6920
There are several good ones out there but if I could only have one that would be my choice. Having had Armalite, Bushmaster, and others, the Colt 6920 is the one I rely upon when I know I'll be putting thousands of rounds down range and cannot deal with malfunctions of less than top quality parts.
You will most likely need to look/purchase online as they are rarely found on the shelf
in retail stores. Traditionally in the M4 platform Colt markets to the LE and military. Before the election yesterday they could still be found in the $1100.-$1300. range.
I watched the price of 30rnd AR and pre-Clinton normal capacity GLOCK magazines rise before my eyes today. Demand is heavy, supply is becoming evermore short, and retailers are taking advantage of the rush before Obama destroys the firearms Industry altogether.
Buy as much ammo as you can afford. Obama's 500% tax hike on firearms excise and ammunition is just around the corner.
Stay safe out there.