What,  in your experience,  is the most accurate piston driven AR-15 available.

Bit of a expansive question because there are a list of things that contribute to its latent precision long before the gas operation system.

- Barrel Length
- Barrel Profile
- Barrel Material & Interior Finish
- Chamber (Wylde > .556  or .223 on its own)
- Muzzle End (Crowning > Muzzle Device)
- Squaring the Upper
- Upper-to-Lower Fit  

As a general statement, start with any manufacturer who offers a Wylde Barrel, melonited, over 14.5" and they match the upper and lower receiver.

Report I have read say the Rugers are shooting well...

I have or have had , HK416 10 inch, HK416 14 inch , LW 10 and 14inch , Sig516 16 inch and an HK MR556.

My HK MR556 shoots the tightest groups by far.

Sorry I don't have numbers to back it up, but these are my observations.

I can get MOA accuracy from my PWS mk116 mod 1 using standard Hornady 55gr FMJBT bullets.

I like the 1/8 of the PWS, and I've done fine with the AA as well.  Seems you can reload to just about any though.

Proarms par mk3 with 18" barrel. One hole 11mm at 100m with factory Hornady 55gr. Less than 1/2 inch at 110yards for you non-metric guys. So better than 0.5MOA

My build with an AA kit and Rainier Select barrel manages an average of about .75moa with 69gr SMKs in front of Varget.

Ultimately the piston really has no bearing on accuracy in my observations.  So long as the drive rod puts no pressure on the block by making contact with it the system is still free-floated, and the projo leaves the barrel before the piston is actuated so there is no effect there.  The belief that piston guns can't be accurate is just silly.

I  have a number of of 556 AR's all with Geissele SSA-E triggers. Among them are Noveske Afghan, LMT CQB, LWRC M6A2 SPR, and a Ruger SR556. Surprisingly the SR556 shined when I put the Geissle in it. Now it shoots the tightest 5 shot 100 yard groups. Usually around 0.6 moa.

This much I can attest to. Many will be pleasantly surprised with the improvement of their groupings (provided their isn't another problem in the platform) after the installation of a quality, smithed trigger.

And it's not just rifles, pistols too. A friend made the tragic mistake of purchasing a S&W Sigma as his first handgun. His groups at 10 yards were watermelons. At 15 they were beachballs with plenty of fliers. He twiddled and fiddled with the rear site. No change.

I ran 2 mags through the gun and could do no better than a pumpkin - the symptom was all the trigger related. I looked at him and said if you want to keep this gun, you're going to need to do something about this atrocious trigger.

After a good trigger job, while things weren't stellar, they were certainly improved. I could manage a Cantaloupe sized grouping at 10 yards with the occasional  flier.

I have replaced the triggers in both of my Ruger piston guns with a National Match and a Timney respectively.  The differences were immediately obvious even to a casual observation; group size shrank by about 1/3.  Triggers do matter.  My FNS40 has an outstanding trigger and shoots 1" to 1.5" groups from a rest at 25 m.  My M&P 40 Shield has a heavy trigger shoots OK but not great.

My Rugers are stellar performers.

+1 on triggers. I put an SSA-E in the MK116 and and SD-E on the Mk216.