For me, the Light Machine Gun, every time.
Specifically the FN Minimi Para, with folding buttstock, collapsable bipod, and M16 magazine feed.
The weapon is versatile enough to cope with almost any post apocalyptic situation, and while it might not be as accurate as a sniper rifle over longer distances, it still has the range to punch a grapefruit-sized hole through a target at a great enough distance that they would be no threat to me. Threat pproaching at long range? LMG. Multiple enemies approaching at speed? LMG. Vehicle attacking? LMG. Area suppression or denial? LMG. Close Quarters? Saw them in half with an LMG. Admittedly, there are guns which can perform each of the above tasks more effectively, but the LMG can perform ALL of the tasks equally well, and with a larger magazine than the combat rifle, reloads are less frequent. The adjustable stock means it can be compacted for ease of storage, transport or when entering tight spaces, the bipod can provide extra stability when manning a defensible position, and the magazine over the belt means you still have one hand free to steady the weapon, and don't have to worry about it snagging or trailing in dirt.