Today I say the .22 because that is what the NRA and the Boy Scouts mandate for new shooters in the rifle program. (Well, technically they mandate rimfire, but I'm not buying .17 HMR for the rifle merit badge kids)
No felt recoil, little noise, cheap, plentiful(at least pre-Newtown), and available in a variety of easy to use, accurate, cheap and plentiful rifle and pistol platforms.
Also, when I went to college and started having to buy my own ammunition, and ammunition for assorted lady friends, I realized the value of my humble Ruger Mk II 22/45.
For what it cost for one box of .38 Spl range fodder, the girlfriend and I could chase around RC and Diet Coke cans behind the fraternity house all afternoon.