A marginal hit with any round will likely not stop a determined aggressor.
Multiple hits, center mass with any round (save mebbe a 22 ) WILL likely stop a determined aggressor
There is no "best" round, there IS skill and training and getting your hits with whatever you happen to be able to shoot well.
While caliber war discussions are indeed fun and can lead to a growth in knowledge, in the end, it's the shooter, not the gun, that will make the difference.
Or not.
With that said, I prefer the 45 or the 357Sig, but would not feel underguned with a 9mm or even a 380 for civilian defensive purposes.
Also, there is the thought that most shooters can "drive" a 9mm faster than say a 40 or 45 or 357, therefore, at some point in the exchange, the 9mm shooter is going to get a whole extra round downrange (and preferably into the target)than a shooter with a snappier or heavier recoiling gun.
(This was brought to my attention by Rob Pincus this past weekend during his CFS/APH class but more on that later)
If I'm going into a fight and know it's coming, Im taking a rifle.
peace