Yup...

AR-15 for all the already stated reasons, and for one that I haven't seen mentioned. The AR-15 is a standard, meaning parts are parts, and magazines are supposed to fit every rifle made. The larger framed AR-10\SR-25\LR-308\whatever else is out there is a collection of some proprietary parts, including different magazines. If SHTF is one of the reasons, I'd be staying with something that has almost universal cross compatibility.