Agamemnon and Clytemnestra's oldest daughter definitely gets the raw end of this deal. Her father sacrifices her to Artemis to atone for his own crime against the goddess. The virginal girl is totally innocent of doing anything bad like ever, so it's a bummer that she's got to die. Ironically, it's Iphigenia's very innocence that makes her a fitting sacrifice for Artemis. Though the goddess was sometimes thought of as a protector of young virgins, many of her cults were know to offer virginal girls as human sacrifices to their goddess.
Interestingly, in some versions of the story, Iphigenia is saved by Artemis at the last second and replaced on the altar by a young deer. Iphigenia is then whisked away to a place called Tauris, where she becomes a priestess of Artemis, and performs... guess what? Human sacrifices. Man, that Artemis just doesn't quit.