If it's a direct gas impingement gun, I'll take anything Colt
makes.... 6520, 6920, etc, as long as it has a lightweight
barrel. They might be a bunch of douches as a company, but
their QC on rifles is generally top notch.
Of course, depending on who you talk to, you're going to get a lot
of "it doesnt matter" type answers, as well. A lot of ARs are so
similar, that for many applications, it doesnt matter much what brand you
choose.
The only brands I won't touch, are hesse, vulcan, and olympic
arms. I'm not one to get entirely political about gun companies, but
I'm still pissed at olympic arms for the "7.62 pistol" debacle that caused the
supply of foreign steel core 7.62 x 39 to evaporate. What a mess
that was... and all so olympic could sell a dumb pistol chambered in
that caliber.
-Mike