Personally, I don't subscribe to the belief that their exists some illusive ideal, universal one-size-fits-all, platform for self-defense. I think it's best for folks to find what works best for them on an individual level, what they shoot best, what fits their hand best, what is best suited for their unique situation, etc.
Full disclosure: as much as I like .357 SIG, it's not my primary carry cartridge of choice, nor do I have any intention to adopt it as such. I carry .40 S&W, which I have found suits me best. I live in an area where humans aren't the only threat, ergo I feel safer carrying a cartridge with a bit more oomph than 9mm.
I think that each cartridge has its own inherent strengths and weaknesses, whichever is "best" is entirely subjective based on the individual who uses it, where they live, and what they might need it to do.
For example, obviously it's better for folks who live in Alaska to carry something more powerful like 10mm Auto or even a Magnum cartridge because they might just as well encounter a Polar Bear as a hostile human.
I also think that the psychological aspect of feeling confident in what you carry is of paramount importance, as obviously you're more likely to remain calm and collected if you believe that you're prepared, so even if it's a placebo effect, someone is much better off carrying what they feel confident with than something that causes them doubt, regardless of whether they doubt the performance of the cartridge or their own ability to use it effectively. So I encourage everyone to carry what works best for them.
As for what's "enough" for self-defense, folks have successfully defended themselves with .22 Pistols. Are their better options for self-defense? Absolutely, but even .22LR out of a Pistol is potentially lethal, so I would say that it's enough. Obviously, you're better off carrying something centerfire because it's inherently more reliable than rimfire ammunition, and more powerful cartridges are more likely to stop a threat regardless of variables like what angle to have to shoot them from to ensure adequate penetration, but if .22LR is all you have or the best you can carry due to some handicap, then I say carry it. Otherwise, I would feel more confident with .380 ACP.