Im not sure if there is a "best" AR on the market.  I think most of your major manufacturers make pretty good quality rifles and parts.  You just gotta find one that's built and configured the way you like it.  I will say that some of the top tier rifle makers do put a little more effort into fit & finish.  However, I don't think you'll see a huge difference in terms of rifle performance.  As far as the Bushmaster/Windham Arms, I'm sure the new company is just using the same factory/tooling as the old Bushmasters.  As long as they have some experienced people there, I'd be willing to bet the quality is on par with the old Bushmasters.  I would look for some online reviews - if others say the quality is there, I wouldn't hesitate to buy one.