If you're looking to define
"the best" then uses come into play. If you don't want to limit the things you can use your handguns for, then there's really only two choices:
the .357 magnum and the .44 magnum. Even if you ignore the Specials, both cartridges can be downloaded to accomplish any small-caliber task, and both can be uploaded to "I'm hungry" levels. Obviously, the .44 wins out at the top end, but the .357's no slouch when it comes to making meat, and a lot of folks shoot it better.
the .357 SIG was designed to replicate precisely one .357 magnum load, in an autoloader. If that one load does what you want, then it's a good choice, but that one load was chosen for effectiveness against human assailants. Handguns are useful for more than just that. The .357 SIG isn't.
So it depends on what you want your handguns to do. If all you're concerned about is self defense against human attackers, you've got at least 10 essentially equivalent choices. At least, the real-world differences are so small that decades of controversy and debate haven't produced a clear winner. Logic says that's because there isn't one. If you want to expand your horizons beyond that limited domain, your choices go down significantly.
If full-spectrum utility is a consideration, you're limited to revolvers. The fact that autoloaders use the ammunition to cycle the action confines such guns to a limited power window. This restricts utility. Likewise, on the high end, revolver cartridges can be loaded to levels that would be impossible or stupidly dangerous in an auto. At the extremes, sixguns rule. But those hand-cannons sacrifice the low-end uses that make the .357 and .44 so versatile.
Is the .357 SIG a good handgun round? I'm sure it is, although I've no interest in owning one. Is it the best? No, it isn't.
--Shannon