9mm, hands down. I have owned and carried .22, .32 acp, 380, 9mm, 40 S&W, 45 acp, .38 Special, 357 Mag & 44 Mags. The 9mm holds more rounds, typically, has lower recoil, is capable of 'combat' accuracy or better, in days gone by ammo was plentiful & inexpensive and with wise selection of defense ammo, as or more capable of most other calibers.
.22 & .32 acp are marginal for defense, especially in the small guns that most people buy in those calibers. .380 is slightly better, but not much. .38 Special 'snubbies' are difficult to shoot (as are most double action revolvers that are not match guns). All but the 9mm & 40 tend to suffer in the magazine capacity area. 357's kick pretty hard for the average person & the 44 is worse & some would say overkill.
So, a 9mm semi auto with standard (high) capacity mags, loaded w/ high quality, reputable hollow point ammo is about as ideal as one can get.
That being said, if I was mag cap. or ammo restricted (<10 round, no HP, etc.) I might consider another gun, probably a 45 acp or 38/357. (or just carry a crap load of extra mags :biggrin: )
I personally carry a Glock 19 &/or 26 loaded with Gold Dot 115 or 124 gr in either non +p or +p ammo (depending on availability). My backups are one or more of G26, LCR357, LCR38, LCP380. Again, they are loaded w/ Gold Dot ammo.
