I would have to say .357. I think I own multiples of almost every caliber. I don't really have a count but I know I have at least a dozen .357 revolvers and one .357 rifle. .22LR would have to be close though, I don't have a count on .22s. Then probably .223/5.56 NATO, I think I have six of those. I probably have around 5-6 9mms, no I take that back, if I count 9mm carbines I have at least 7 9mms. I don't know, but .357 surely takes it for me.