[Kamog]

I have two favorite episodes from the original series.

Episode 2: Where No Man Has Gone Before
(The one in which they go to the edge of the galaxy and two of the crew members get powerful psychic powers.)

Episode 37: The Changeling
(The one in which they encounter the powerful 'Nomad' probe)

Well, the third favorite is:
Episode 24: Space Seed
(They find Khan and his group in the ship S.S. Botany Bay)