#20 THE CORBOMITE MANEUVER - The ultimate bluff by Kirk, unlike what General Order 24 was in “A Taste Of Armageddon.”
#19 THE DEVIL IN THE DARK - Jaws or Alien[complete with the eggs]. A fascinating non-humanoid alien race.
#18 THE IMMUNITY SYNDROME - This rivals “The Doomsday Machine” or V'Ger cloud from Star Trek: The Motion Picture.
#17 LET THAT BE YOUR LAST BATTLEFIELD - Engaging anti-racism statement.
#16 THE MAN TRAP - Vampire-like creature story with salt replacing blood which was the need in the “Obsession” episode.
#15 THE ENTERPRISE INCIDENT - Klingon D-7 used as Romulan D-7 and a great follow-up to the Romulan episode “Balance of Terror.”
#14 THE TROUBLE WITH TRIBBLES - Space Station K-7.
#13 JOURNEY TO BABEL - Federation diplomats, Spock's parents and murder on board.
#12 MIRROR,MIRROR- Mirror universe. Spock is logical there too.
#11 DOOMSDAY MACHINE - This rivals the V'Ger cloud of Star Trek: The Motion Picture in which we see Commodore Decker's son.
#10 THE CHANGELING - This rivals the V'Ger/Voyager of Star Trek: The Motion Picture finding the creator.
#9 SPACE SEED - Khan v. Kirk , leads to Wrath Of Khan and rebooted Into Darkness.
#8 THE ULTIMATE COMPUTER - Multiple Constitution Class starships and a M-5 that rivals HAL9000 or Omega[Ark II].
#7 COURT MARTIAL - The ultimate court case and mystery.
#5 A TASTE OF ARMAGEDDON - General Order 24, belonged in the "Mirror, Mirror" universe, but if kept would have made a different Federation in TOS...canceling the Prime Directive.