The obvious answer, as most have perceived, is the .357 Magnum, used often with its understudy ammo, the .38 Special.
The .44 and .45 guns have their points, but they're too bulky for much carry, as is their ponderous ammo. Great in certain circumstances, but not outstanding all-round.
The 9mmP is probably my choice in auto calibers, likely in the CZ-75. But the .357 is best, really
Lone Star