223/5.56 Nato for the reasonable prices now and widespread availability because so many other have it; 22 LR for the same reasons, and 12 ga shotgun. 308/762 Nato is good, but pricey now. 338 is fine but on the high end. Anybody who buys oddball calibers and gauges for SHTF purposes is not thinking straight.
Ah yes, forgot this one—7.63x39 AK, perhaps the best.
Common is in; oddball is out and stupid to deal with in a SHTF situation. The empty weapon will be a useless doorstop soon enough.