I have acquired a few A2 stocks the last few days so will pass on the info. I HATE cheap stocks so always like to find quality.

Genuine Colt U.S. Army M-16A2 stock, very good condition, this is always the baseline.  

2 new DPMS stocks for $65 each at the gun show.  I have a good eye for materials and quality and I can't tell them from the Colt military stock.  Probably from same supplier.  Metal trap door is black vs. what looks like gray zinc phosphate on the old Colt military stock.  Other than that same plastic, same foam filling, outstanding quality, stainless buffer spring, mil-spec buffer, metal parts good.  Only downer is the buttplates don't fit the stock quite as nicely as the Colt.

Rock River Arms, bought 2:  Stock part polymer including foam filler appears same materials as DPMS and Colt (stock is solid) but buttplate trap door is polymer not metal.  However it is a very high quality rigid polymer and not cheap and flimsy as other "plastic" trapdoors I have seen.  Buttplate to stock fit is best of any A2 stock I have owned.  Also the recoil spring is stainless ala mil-spec and the buffer appears mil-spec, and the metal parts and screws appear mil-spec.  At $60 shipped on sale right now the best deal out there.  I actually pulled a used Colt buttstock off one of my A3 rifles and replaced it with a Rock River Arms…it is that nice.

Windam Weaponry:  Same as DPMS to me. Mil-spec IMHO.  
Bushmaster:  Same as WW, DPMS

BTW, "Mother's Back to Black" auto trim restorer makes the gray look really good and almost black.