556
The 556 is a great gun. It could have been the greatest though if the CEO didn't veto the original plans to just make a 16" barrelled 552 and call it the 556. Word is that he had issues with the folding stock model as well, at least according to the Sig reps I talked to.
The 556 is piston operated, and is basically a very accurate 5.56 AK. You don't get the high temps on the gas tubes when firing at high rates like in ARs, and the gun itself comes out of the box more accurate than the best DPMS.
The only reason I still have my AR is because I have tricked it out with close to 2 grand in mods, plus it can fire 458 socom with me just switching out the upper assembly. My 458 is a great Grizzley gun back in MT, because it hold 10 rounds of what I can describe as 45-70 +P rounds in a mil-spec 5.56 30 rd magazine. No change in followers or springs.
If Sig ever came out with a 458 upper, my AR would be gone faster than crack in Marion Berry's house. Great gun that 556 is.