Agreed with all the prior posters. I consolidated down to one ammo caliber for my pistols awhile back, and it was 9mm. I'm perfectly confident a quality 9mm HP round (I use the Speer Gold Dot 124g +P JHP mentioned before) is a good self-defense round. I also went with 9mm for more inexpensive ammo (relative) and greater capacity.
One other item. I've found the following image to be telling - while there are MANY factors that go into whether a round will do it's intended purpose, this is an interesting image :
