Without any disrespect to any of the other fine rifles listed, I'd have to vote for the FAL, and here is the logic:
I was a Marine Infantryman, I've broken AR/M16 pistol grips, firing pins, dinged mags to the point that they won't feed (This was done as a boot, private and PFC, after which I accepted the rifle's need for a lighter touch). and comparatively speaking,the FAL is extremely strong.My FALs have been abused, left without cleaning for thousands of rounds and been generally neglected as a proof of concept and have kept on trucking. It has a forged receiver (DSA) and is generally very forgiving of abuse.
The FAL is cake to scope with the railed dust covers. One of mine wears an Aimpoint, the other a Nikon, rock-solid mounts, with no zero issues.
A MAJOR factor in my choice is that a complete and utter moron can replace any assembly in the rifle, there are basically no depot-level repairs needed for the rifle, save replacing the barrel/headspacing. The locking shoulder is replaceable to allow for wear, as well as the ejector block. It is completely modular, as you can swap a longer barrel upper receiver and bolt in the time it takes to unscrew the hinge pin. This is a major plus in a "Battle Rifle" because you may not have a "depot" or an "armorer" to help out if your rifle goes down.
Another huge plus is the availability of spare parts. The sheer number of rifles out there means that the price of these parts are generally pretty low. I bought two whole sets of "guts" for my girls for about $200.00, that's every internal part of the rifle, and spare lowers and buttstocks. Running rifle=combat effective. I have friends who love the M1A but finding good GI parts is a bear and expensive. The bolt must be lapped in and headspace checked by a gunsmith, so if one breaks you are less a rifle. Also, I'll echo the PITA of mounting a scope on the SAI receivers, which were found to be out of spec on more than one of a friend's rifle's. One receiver was also out of spec in the area where the op rod travels, which was peening the receiver.
That being said, the M1A is a dream to shoot, the irons are second to none, and it has certain magic all it's own. But not my first choice for a battle rifle. The HK91 is really a cool rifle, but for running and gunning, in the real world, the lack of a BHO, and the ridiculous charging handle, place it in slow-moving zombie rifle territory, for me, anyway.