I've had pretty much all of them (except JP, which is a Nordic BCG with John's barrel) and the the "best" for pure accuracy, has been the CLE, the Nordic was 2nd. BUT that could be barrel specific. The CLE will give you the best barrel on average (and he will fix it if it's not) and you may or may not get a hammer with Nordic, (prob 75% good, 25% OK but still better than all the Ciener's)
Stay away from all the Ciener style conversions (CMMG, TacSol, Spikes, etc) for pure accuracy, good guns can be built on these, but you're not going to just buy one and get a hammer, on average.
The BCG's in the CLE (which is a military unit) and Nordic are also much more reliable and not needed to be tuned and tweaked like a Ciener style to function properly with match type ammo. These two styles of BCG also seem to damage the rounds less as they load into the chamber.
Over the years, I have personally had several CLE, Nordic, DPMS, CMMG, TacSol, Colt, JA Ceiner, Spikes, m261 (and probably a few more I can't remember), just so you know I'm not blowing smoke.