This debate will go on for as long as there is more than one company building AR's.
As to whether brand X, Y or Z is better, I don't have a clue.
My first AR was a DPMS Panther Bull 20 Fluted. I had a 4.5x14x40mm Nikon Buckmasters Riflescope mounted on it. This AR was rather loose in regards to fit and finish, especially between the upper and lower receivers. However it would shoot sub 5/8" groups just about every time I took it to the range and a lot of the time it was sub 1/2" at 100 yards from the bench.
My next AR was a Bushmaster 16" M4 ORC (optics ready carbine) with a chrome lined barrel. I had always heard (or should I say read) that chrome lined barrels would not produce the accuracy of a good Chrome Moly or Stainless-Steel Barrel. Well you couldn't prove it by this Bushmaster Carbine as most 5 shot groups are in the 3/4" range and all sub MOA at 100 yards with a 1.5x4.5x32mm Bushnell Elite 3200 Riflescope. This fit and finish on this Bushmaster is great and the upper to lower fit is nice and tight.
My next AR was once again a DPMS, this time a Panther Bull 20. It is set up with a 4x12x40mm Bushnell Elite 3200 Riflescope. This one has much better fit and finish than my first DPMS and the Upper to Lower fit is much tighter. This one hasn't broken the 1/2" barrier as yet but most groups are right at 5/8", so regularily it is boring.
A guy I work with from time to time has a DPMS .223 Sportical. We swapped out the handguards for a Free Float Tube and did a trigger job on it. Shooting the same ammo I have been using in all of my AR's this Sportical will print 5 shot groups at 150 yards in the 1" range (usually slightly less than 1"). The owner couldn't be happier with his purchase.
I have a buddy that has a couple Olympic K8 Targetmatch Upper Receivers. One is on an Olympic Lower and the other is on a Stag Lower. Fit and finish is good on both and both shoot about like my DPMS Bull Barrels.
Truth be known I have been doing trigger jobs on AR's for friends as of late. I have worked on Bushmasters, DPMS, Olympic, Stag and Armalite. For some reason it is easier to get a good trigger on the DPMS than the other brands.
I don't have all the answers, but do know what has worked for me. My Bushmaster doesn't get cleaned as often as it should, and it rides in a vehicle every day in an overhead gun rack yet I know when I squeeze the trigger it is going to fire and fire every time as so far it has never missed a beat.
I think every company out there today can make a bad example from time to time.