Not what do you carry, what do you think is "best." Please, comment on WHY you think it's best. Given all the different uses handguns can be put to, I think the reasoning should be interesting. If I leave a caliber off, write it in and tell us why it's better than the ones listed! I've only got so many options I can include.
My vote for "best" is .357 Magnum. It's unparalelled as a defensive round, it can also be loaded to be great for hunting small to medium game, provide protection against nasty critters up to black bears, great for target shooting, can even be used to great effect in competition. I'm not even adding in the fact that guns chambered for this caliber can also shoot the .38 Specials. Just the .357 all by itself can do all the above. It is, IMHO, the most versatile round out there, and doesn't sacrifice excellence to achieve that versatility. Handloading really makes the most out of all the different things a .357 can do, and be great at.
If were to be limited to one handgun only and it was to serve multiple purposes, then I too would go the .357/.38 Spec route. However, my EDC is an Officers Model 1911 in .45ACP. The .45ACP round has stood the test of time and with either 185 or 230 grain jacketed HP loads, it's a proven fight-stopper.