Best AR for what? If it is for occasional range trips and taking up room in the safe then any number of manufacturers would be fine. If you have more serious uses for it like self defense or if you plan on taking carbine courses with it then you will want to something better than the typical RRA, Bushmaster, Stag, DPMS types.
BCM is the easy answer. They have a ton of options, excellent quality, and their prices are fair. Colt, Daniel Defense, LMT, and Novekse are also excellent. These companies build ARs that more or less comply to the known best practices around building ARs. Many people don't consider how important little things like gas port spec, the weight of the reciprocating recoil parts, the type of pins used in the front sight base, and chamber dimensions can be.
If you can't quite spend on Colt, BCM, Noveske, Daniel Defense, LMT, etc then I would look at two places: Spikes Tactical or Smith and Wesson. Of the two, I prefer Spike's for several reasons, not the least of which is their use of the superior 1 in 7" twist rate. S&W uses mostly 1 in 9".
An AR is pretty big investment. I suggest learning as much as you can. Check out
Vuurwapen Blog and
Tactical Yellow Visor for more detail than you will get on a typical forum.