M5 and Nomad, just to see which of them could take control over the Enterprise, with or without destroying the other, in the Mirror Universe, while battling the Doomsday Machine with the Enterprise.
Also, Gary Mitchell (after the accident at the edge of the galaxy) and the Talosians.
Navigator NCC-2120 USS Entente
Stokely: In an insane society, the sane man must appear insane.
Harvey Holroyd: Where'd you get that?
Stokely: Star Trek. [leaves the room]
Harvey Holroyd: [to himself] God, I miss that show. (Source: "Serial", 1980.