Quoted:
Looking for the best (readily available) A2 upper.
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Check out Windham Weaponry. They have a variety of A2 carbine and rifle barreled assemblies.
ETA: My mistake.
What you may want to do is get a complete A2 upper receiver, a Windham Weaponry A2 Gov't profile barrel assembly, Delta ring assembly, and gas tube and assemble your own.
Barrel
http://www.windhamweaponry.com/shopexd.asp?id=309#axzz3TB9cn1sr
Gas Tube/pin
http://www.windhamweaponry.com/shopexd.asp?id=209#axzz3XmlWW1aD
Charging Handle
http://www.windhamweaponry.com/shopexd.asp?id=209#axzz3XmlWW1aD
A2 FH
http://www.windhamweaponry.com/shopexd.asp?id=14#axzz3XmlWW1aD
Crush washer
http://www.windhamweaponry.com/shopexd.asp?id=16#axzz3XmlWW1aD
Delta ring assembly
http://palmettostatearmory.com/index.php/ar-15-05/upper-parts/ar-15-delta-ring-assembly.html
RRA Forged A2 Upper Receiver Assembly
http://www.rockriverarms.com/index.cfm?fuseaction=category.display&category_id=279
All you will need in addition to these items is the M16 BCG of your choosing.