Ratings according to the imdb.com

TOP 10

(1) The City on the Edge of Forever (episode 1.28) rating: 9.6
(2) Balance of Terror (episode 1.14) rating: 9.6
(3) The Doomsday Machine (episode 2.6) rating: 9.2
(4) The Trouble With Tribbles (episode 2.15) rating: 9.2
(5) Amok Time (episode 2.1) rating: 9.2
(6) Mirror, Mirror (episode 2.4) rating: 9.1
(7) Space Seed (episode 2.4) rating: 9.1
(8) The Devil in the Dark (episode 1.25) rating: 8.8
(9) The Menagerie: Part 2 (episode 1.12) rating: 8.8
(10) Journey to Babel (episode 2.10) rating: 8.8

BOTTOM 10

(10) Elaan of Troyius (episode 3.13) rating: 6.3
(9) Plato’s Stepchildren (episode 3.10) rating: 6.3
(8) The Lights of Zetar (episode 3.18) rating: 6.1
(7) Whom Gods Destroy (episode 3.14) rating: 6.1
(6) For the World Is Hollow and I Have Touched the Sky (episode 3.8) rating: 6.1
(5) Turnabout Intruder (episode 3.24) rating: 5.9
(4) The Cloud Minders (episode 3.21) rating: 5.7
(3) And the Children Shall Lead (episode 3.4) rating: 5.0
(2) The Way to Eden (episode 3.20) rating: 4.6
(1) Spock’s Brain (episode 3.1) rating: 4.5