I would say the AR for SHTF is one that YOU personally have ran hard and had no failures. IF your AR will run a 1000 or 1200 in three days in a class with no malfs then you can probably trust your life to it. The AR problem children that I have had experience with will generally show you thier issues in that type of environment. If you do not take a class or, have a range that allows you to run the gun hard in drills, putting 3-4-5 mags through in a few minutes, and you have to take a rack gun, right that second, Go Colt, they have less issues than others, as a general rule.
No matter which gun you have, learn how to clear malfs, and do PM on the weapon. Springs, extractors, and bolts are cheap. Have spares, know how to change them and troubleshoot which item might be the issue.