I will echo the single most important part of this thread...
IT HAS TO BE 100% RELIABLE IN YOUR GUN!!
That being said, most modern self-defense rounds will do a pretty bang-up job of ruining the Bad Guy's day (pun intended). If not restricted in your area, any modern hollow point or expanding tip (EFMJ) ammo will do the trick. Some already mentioned are
Hyra-Shok
Gold Dot
Golden Sabre
XTP
Starfire
Ranger
Pow-R-Ball
And some of the less well-knowns like
Aguila IQ
MagSafe
BeeSafe
Aside from the reliability issue, it all comes down to shot placement. If you can't hit the BG, then the biggest gun in the most powerful caliber is nothing more than an expensive, noisy club. Suppose you had the choice of A) Missing the BG completely with a .500 Smith, or B) Hitting them where it counts with a .22 LR. Which do you think will be more effective?
Safe shooting!
Brad