Currently, "Day of the Dove." One of the rare really strong episodes in Star Trek's wobbly, mercurial third season. Michael Ansara is delicious as the Klingon antagonist Kang, chewing the scenery like a boss. While its basic message of escalating hatred and violence is a little on the ham-handed, non-subtle side, this episode does have lots of poignant, intense moments of it. Especially when the human characters got so enraged they were starting to turn against each other. With some great quotes as well. Definitely worth watching for its true Star Trek flavor.
Chekov: You Klingon monsters, you killed my brother!
Kang: And you volunteer to join him; now THAT is loyalty.
Chekov: You Klingon monsters, you killed my brother!
Kang: And you volunteer to join him; now THAT is loyalty.