RE: best self denfense gun
If I were to rank them it'd go like this...
1.) .357 Magnum (if you prefer a revolver) with any premium defensive JHP or frangiable bullet (i.e. Glasier Safety Slugs).
2.) .45ACP (if you'd rather have a semi-auto), preferably with 230gr. JHP's. 185gr +P's are OK too.
3.) .40S&W
4.) 9mm Luger (9x19)
5.) .44 Special
6.) .38 Special
7.) .380ACP
I can't recommend the 10mm or the .44Mag. Both of them just have more recoil than most people can handle in a handgun small enough to be practical for carry or to keep in a desk or nightstand. The flash and muzzleblast are also a bit much as well. Since the gun will most likely be used at night, muzzle flash is an important consideration because you don't want your first shot to blind you. Overpenetration is also an issue with these high powered rounds.
Are you looking for a carry gun or a home defense gun?
If the gun is only for home defense then I'd stick with #1 or 2. I'd also suggest that you consider a 12ga. shotgun for the house. Shotguns shooting buckshot are vastly superior to any pistol for short range firepower, are easier to aim and shoot under stress, and with the right loads overpenetration isn't too great of an issue.
Mike