On a per round basis, based on how many I actually shoot, .22lr.
Cheap is good. Mild shooting characteristics are good. Consistently decent accuracy is good.
Not saying that it is a caliber that I'd choose to defend myself or my home, but it is the round I'd choose to train with.
Quality trigger time equals proficiency. Lower cost per round equals more quality trigger time. .22lr equals more quality trigger time.
If it came down to the caliber I'd choose to have with me if society were to break down, then, it would have to be .22lr. Specifically, it would be my scoped .22lr rifle (make and model withheld). The reasons for such probably doesn't meet the requirements of this particular site, enough said.