As part of my job I occasionally have to empty the trash barrels at a public shooting range. Being a scrounger, I pick up whatever decent, rifle brass I come across. Not counting rimfire, pistol, and military brass (.223, 7.62X39, 7.62X54 rimmed), the most popular calibers seem to be:
.30-06 Sprgfld
.30-30 Win
.270 Win
.25-06
.243 Win
.22-250
Without a doubt, .30-06 and .30-30 are the most abundant this time of year. Still see a moderate amount of .243 Win, but .270 and .25-06 are more common. This fall I've come across one 6mm Remington case.
What really amazes me is how many people must be out there shooting Russian military stuff (7.62X39 and 7.62X54 rimmed). Don't know if they hunt with these or not, but I'd take a .243 Win or 6mm Rem over them any day.