I saw a new custom .510 revolver in a gun magazine a few weeks ago, maybe THAT'S the new best caliber....
Seriously, I'm not a fantastic shooter by any means, so I agree with all those that have said "whatever you can hit best with", if you can't hit the broad side of a barn with anything bigger than a .32, carry a .32 and work your way up.
A handful of .32's center mass is better than a miss with a 9, .40, .45 or whatever hand cannon you can't shoot.
