Always on person:
Glock 17 Gen 4 - There's not always time to grab a carbine or shotgun, but a reliable pistol is better than no gun.
If there's time:
Colt AR-15 6520 or 6720 - This is my first choice. Less chance of penetration through exterior walls with the AR-15 carbine than with the pistol or shotgun. The 55 grain FMJ bullets fragment and tumble, and I have 30 rounds in my PMAG. Light, accurate, compact, works well with optics, has more cartridges in the magazine than the pistol and shotgun combined - what's not to like?
If the AR-15 is down or my wife has the AR-15:
Remington 870 Police Magnum - I have normal 9 pellet 00 buckshot in the tube and 1 OZ slugs on the side saddle. Last choice because it requires 2 hands to operate, has the fewest cartridges ready to go, and is manual. That said, mine was super smooth from the box, patterns consistently with Remington and Winchester buckshot, has rifle sights, and has never failed me.