M1A. I'd prefer a military M14. Not because of the FA, but because of the quality of the receiver. I'd take it over an FAL, but not by much. And it's a close call between them and a good bolt-action (Lee-Enfield, Mosin Nagant, Savage 110, '03A3 etc). Slower rate of fire, but more reliable. And ammo in stripper clips is lighter than ammo in mags.
I suppose a good suppressed .22 would be ideal,
if you initiated the action. One could scavenge a rifle from the corpse of one's enemies. Of course, a suppressed pistol (or a knife, garrote, hammer, pipe, hatchet, screw driver or any other "discreet" weapon) might be even better, since they could be concealed easier.
[Dons Nomex suit] I'd take almost ANY rifle, including WWII mil-surps, over the AR. IMO, it is flawed by design. It is simply too finicky. Maybe an AR
style weapon with a different gas system, but it'd still be at the bottom of my list. I'd recommend the following site to see what I mean about the design.
http://www.madogre.com/Interviews/Hate_the_AR15.htm