Every Mass Effect 2 Loyalty Mission - Ranked
6. Samara - The Ardat-Yakshi

Though Mass Effect has always had choices, this one is on another level. You can choose to kill Samara and add her daughter Morinth - a sex vampire - to your team. This is a strictly non-combat mission, but in a way it's the most violent of the lot.
The key here, unbelievably, is douchebaggery. Once you've made enough of a scene, Morinth invites you back to her swinging bachelorette pad. Here, we see Shepard weaker and more vulnerable than usual. Morinth takes control. She flatters us, goading us to merge minds. It won't kill us; we're too strong.
Samara bursts in then to kill Morinth and finally end her daughter's reign of terror. Siding with Samara ends the mission then and there: back to the Normandy you go. Siding with Morinth though will see her kill her mother, then assume Samara's place on the crew, with nobody but you any the wiser. But it essentially means murdering a Jedi because you have the hots for a serial killer. Oh, if you try it on with her again, you get a Game Over. Because she's a sex vampire, idiot.
Bonus; Shepard’s dancing can feature in this mission, which definitely helps its ranking.