9mm
cheap
low recoil
easy to buy
if you can't kill something with one bullet, then shoot some more bullets.
technically I can only buy long guns as of now, and California law has banned all the pistol-carbines. But I would like to have a Ruger carbine in .44mag and a Hi-Point 995 (9mm).
when I can buy a pistol, I would buy a surplus 7.62x25 pistol and a Glock 21 in 45ACP.
7.62x25 has low recoil, cheap ammo, excellent penetration and many of the pistols are cheap, reliable, simple.
The .45ACP packs a deadly punch, still affordable, and the GLOCK 21 is a indestructible, highly reliable, popular (but ugly) pistol. the glock costs more than most pistols, but its more accurate, reliable, durable, indestructible than other pistols.