I really liked the Assimilation mode from the expansion pack for Star Trek: Voyager - Elite Force: Everybody starts out on team Starfleet except for one randomly selected person who becomes the Borg Queen. The Queen can teleport, has regenerating health, and uses a one-hit kill attack which assimilates the victim and turns him into a Borg Drone who fights for the Queen. It gets really tense as your teammates one by one get assimilated and start fighting against you, and the hunters becomes the hunted.