The ar-15 if a long term shtf the police and military will be using this battle rifle so you have ammunition and parts at your disposal short term I would have to say a good little shotgun would do nicely my Benelli M4 should do the trick
If you are defining a Battle Rifle as a FULL power semi-auto rifle (minimum caliber .308/7.62 X 51), then I would say the HK-91 w/the CETME a close 2nd. No gas system to foul or need adjustment, accurate, reliable and easy to maintain. I have 2 of these.
If you are going to allow 5.56mm into the fray, I would say the AR-15 platform. Available parts, magazines and ammo. I have 2 of these.
I also have 2 AK's so I can cover all my bases. I rely on the HK for the primary shoulder weapon. You cannot beat the 7.62 X 51 for penetration and "stopping power".
Mine is my Superior Arms S-15 .223 carbine. I have a Millett DMS-1 Tactical Scope mounted on it. I just LOVE it for general fighting purposes. It lets me have the advantage of a red dot (donut) that I can use for quick shooting needs and it lets me have a variable power scope that goes from 1 to 4 power. My S-15 and scope will let me easily engage targets from 0 to over 400 meters which outdistances my SAR-1 rifle in 7.62 X 39. I can carry a lot more ammo for my S-15 than I can for any of my old M-1A rifles, AR-10 or FN/FAL rifles. For my needs, my S-15 isn't perfect but it'll do until I find something better for now.