Mine is probably during Fallout New Vegas when I took Boone with me into the deathclaw infested quarry. It was one heck of a run and gun battle once we made the mother deathclaw mad just as he ran out of AMR AP ammo.
Another that gets honorable mention, a recent session of Fallout 2. The preacher from NCR asks me to deliver a suitcase to John Bishop in the city of New Reno. So I go do that and on the way out of the Shark Club, his daughter Angela comes onto me so we go do that little encounter. The only way out of the level is through John's penthouse. He snaps and says that either I can go assassinate someone for him or I can go die. I choose gunfire. So I kill my way to the second floor and there's Mrs. Bishop. Yep, romantic encounter with her too. Get down to the ground floor where I left my companions and sure enough, another gunfight. So I completely destroyed the Bishop family and slept with his wife and his daughter both. I don't know if you can own someone more thoroughly than that.