I haven't had time to go through this whole thread, and this may very well have said before. If that's the case let me, as a professional firearms instructor, echo the others' good comments:
The best single firearm for home defense is a handgun that the user is comfortable with. You only need one had minimum to operate it, leaving one hand to control your kids or other people under your protection, operate doors and light switches, or to accomplish any other task that might need to be done. You also have much more dexterity with a smaller weapon, and don't have as many problems around corners and in confined spaces.
A handgun should be selected for ergonomics and comfort before anything else. Don't ignore important aspects like caliber or function, but it should first and foremost be comfortable in the shooters hand. They will shoot better, and will shoot more when they enjoy shooting it. It's like a shoe. Some will fit and some won't. Don't get shoes that don't fit. Then think about caliber. 9mm is probably the best choice for a new shooter, and with hollow point ammo will have plenty of stopping power. Also the dangers of over-penetration and ricochets are greatly reduced. The myth about 9mm being ineffective comes from FMJ ammo.
If one is not worried about concealability then err on the side of magazine capacity and comfort of grip. A picatinny rail and a light are highly advisable. Lasers, however, are overrated, but not without their uses. When something goes bump in the night, a concerned home-owner has only to secure one object from its place of safe storage, not several. You can't always get to the light switch in a dark room, and you don't want to have to grope for things in a stressful situation. Having your light with you will allow you to selectively illuminate a room at will, and will allow you to identify a moving object in the dark that may turn out to be your dog, your kid, your wife, your drunk friend on the couch, or an armed assailant that has broken into your house with malice on the brain.
Anyone with specific questions should feel free to PM me at any time!