The handgun cartridges I find useful are:

22LR - training and small game
380Acp - when you can't carry anything else (and BUG)
9mm - when you need lots of rounds
45Acp - preferred self defense option

10mm - best handgun cartridge IMHO (when loaded to real 10mm level unlike 99% of factory ammo). It shoots flat to 100yards, has the recoil of a 40SW, is ballisticaly superior to a 357 Magnum, enough velocity and power for use against 2 and 4 legged predators, is a cross over hunting round, and lastly...is available in a 15rd Glock.