For most people, Pistol or Bullpup rifle in 5.56 with 32gr high-velocity ammo. Short weapon for maneuverability, 30 round magazine so you won't run out of ammo easily, 32gr HV ammo so if you do miss it doesn't over-penetrate too badly.
But I live in a more remote home, so I use 40gr as it is easier to get and still tends not to over-penetrate all that badly, which is still a minor concern for me.
I use a 20rd mag because I think it is more maneuverable for house clearing corners. I switch from right handed to left handed and find the smaller mag easier when doing so. Also, I tend to (incorrectly) hug cover and will naturally tilt the AR/SBR when leaning out, so I don't want the mag hung up on the corner.