Im thinking of works like Piece of the Action, Tribbles, I Mudd etc, something made to be a funny for the entire episode. While there were occasionally funny situations and exchanges between characters in the other series, did any of them have outright comedy episodes? The DS9 Tribble episode comes closest, but they are revisiting a TOS story. Even in the movies, the only one made as a comedy is Trek4, with the TOS Cast. The other series all had (mostly) talented actors, TNG especially had some funny people in its cast, Spiner had been on Night Court prior to Trek. So why are the comedies unique to TOS?