Yen's picks of "Crucial Episodes". Episodes I consider important are episodes with important plotlines, and episodes where important items are introduced.
Season 1.
1. Crybaby Usagi's Magnificent Transformation
3. Mysterious Sleeping Illness! Protect the Girls' Hearts in Love
8. Is the Genius Girl a Youma? Brainwashing School of Terror
9. Usagi's Misfortune! Watch Out for the Rushing Clocks
10. Cursed Buses! Fire Senshi Mars Appears
13. Girl Power! The End of Jadeite
14. A Powerful New Enemy! Nephrite's Evil Crest
17. Is Usagi a Model? The Focus of the Youma Camera
19. Usagi is Thrilled! Tuxedo Mask's Love Letter
22. Romance Under the Moon! Usagi's First Kiss
23. Wish Upon a Shooting Star! Naru's Pure Love
24. Naru's Cry! Nephrite Dies for Love
25. Jupiter, the Brawny Girl in Love
28. Illustrations of Love! Are Usagi and Mamoru Getting Closer?
32. Umino's Resolve! I'll Protect Naru
33. The Last Sailor Senshi! Sailor Venus Appears
34. The Sparkling Silver Crystal! The Moon Princess Appears
35. Memories Return! Usagi and Mamoru's Past
36. Usagi is Confused! Is Tuxedo Mask Evil?
42. Sailor Venus' Past! Minako's Tragic Love
44. Usagi's Awakening! A Message From the Distant Past
45. The Sailor Senshi Die! The Tragic Final Battle
46. Usagi's Everlasting Wish! A New Reincarnation
Season 2
47. Moon Revived! The Mysterious Aliens Appear
48. For Love and Justice! A Sailor Senshi Once Again
49. For Whom is the White Rose? The Moonlight Knight Appears
51. A New Transformation! Usagi Powers Up
58. The Lovers' Hearts Disagree! The Angry Makaiju
59. True Love Awakens! The Makaiju's Secret
60. An Angel? A Devil? Mysterious Girl From the Sky
61. Huge Shock for Usagi! Mamoru Declares a Breakup
64. In Search of the Silver Crystal! Chibiusa's Secret
68. Protect Chibiusa! Clash of the Ten Warriors
69. Awaken, Sleeping Beauty! Mamoru's Distress
72. Cold Hearted Rubeus! The Four Sisters of Sorrow
73. A UFO Appears! The Sailor Senshi are Abducted
74. Defeat Rubeus! The Final Battle in Space
75. A Mysterious New Senshi, Sailor Pluto Appears
76. Magical Power of Darkness! Esmeraude's Invasion
77. Our Feelings are the Same! Usagi and Mamoru in Love Once Again
82. Journey to the Future! Battle in the Space-Time Corridor
83. The Shocking Future! Dimande's Dark Ambition
84. Wiseman's Evil Hand! Chibiusa Disappears
85. The Birth of Black Lady, the Queen of Darkness
86. Saphir Dies! Wiseman's Trap
87. Believe in Love and the Future! Usagi's Determination
88. The Final Battle Between Light and Darkness! Love Sworn to the Future
Season 3
90. Premonition of World's End? Mysterious New Senshi
91. The Rod of Love is Born! Usagi's New Transformation
92. A Beautiful Boy? The Secret of Haruka Tenou
94. Protect the Pure Heart! A Three-Way Battle
98. Save Friends! Moon and Uranus Join Forces
101. Usagi in Tears! Glass Shoes for Her Birthday
102. The Pure Heart Stolen! Usagi's Biggest Crisis
103. The Arrival of the Tiny Pretty Senshi
106. The Bond of Destiny! The Distant Days of Uranus
109. A Time of Shock! True Identities Revealed to One Another
110. Death of Uranus and Neptune? Talismans Appear
111. The Grail's Divine Power! Moon's Double Transformation
112. Who is the True Messiah? Chaos of Light and Darkness
115. Shadow of Silence? The Pale Glimmer of a Firefly
119. Awakening of the Messiah of Silence? Stars of Destiny
120. Invasion From an Alternate Dimension! The Mystery of Mugen Academy
123. Shadows of Destruction! The Awakening of the Messiah of Silence
124. The Horror of the Approaching Shadow! Eight Senshi in a Tough Battle
125. A Bright Shooting Star! Saturn and the Messiah
126. A New Life! Time for Separation of the Destined Stars
Season 4
128. Meeting of Fate! The Night Where a Pegasus Flies
129. Super Transformations Again! Pegasus' Power
130. Protect a Mother's Dream! The New Attack for Double Moon
SuperS TV Special (Optional Required viewing. lol)
133. Artemis' Affair? A Mysterious Kitten Appears
135. Hearts That Communicate! Chibiusa and Pegasus
143. Time to Believe the Pegasus! The Super Transformations of the Sailor Senshi
148. Shadows of Great Evil! The Trio is Cornered
149. Mirror of Dreams! Amazons' Final Stage
150. Amazoness! The Nightmare From Behind the Mirror
151. The Power Explodes! Ami's Song From the Heart
152. Burning Passion! Mars' Furious Deadly Attack
154. Confrontation in Dreams! Minako and Makoto's Broken Friendship
157. Pegasus is Gone!? Swinging Friendships
158. The Secret of Pegasus! The Handsome Guardian of the Dream World
159. Chibiusa's Little Rhapsody of Love
161. Coming Terror! The Dark Queen's Evil Approach
162. The Center of the Darkness: Dead Moon Circus
163. Labyrinth of Mirrors! Chibi Moon Captured
164. The Golden Crystal Appears! Nehellenia's Spell
165. Time for the Crystal to Shine! Beautiful Power of Dreams
166. Dreams Forever! Light Throughout the Sky
Season 5
167. A Nightmare Scatters the Flowers! Return of the Dark Queen
168. The Awakening of Saturn! The Ten Senshi Come Together
169. Evil Mirror Under a Curse! Mamoru in a Nightmare Trap
170. The Fateful Night! The Agony of the Sailor Senshi
171. For the Sake of Love! Endless Battle in the Dark World
172. Moon Love Power! The End of the Nightmare
173. A Goodbye and an Encounter! The Destiny of Flowing Stars
174. A School Storm! Transfer Students are Idols
176. The Figure of a Fighter! A Shocking Super Transformation
177. Dreams and Romance on a Star! Taiki's Transformation
178. Luna Saw it! The True Idol Yaten
180. Brightness of Calling Stars! Haruka and Michiru's Entry into Battle
181. Seiya and Usagi's Nervous Date
182. Invasion From Space! Seiren Comes Flying in
185. Taiki Sings with Excellence! A Song Carries a Believing Heart's Wish
187. The Power of a Shining Star! Chibi Chibi's Transformation
188. An Invitation to Terror! Usagi's Night Flight
189. Torn Between Duty and Friendship! Confrontation Between Sailor Senshi
190. Truth Revealed! Seiya, Taiki and Yaten's Past
193. The Silver Crystal is Stolen! Princess Kakyuu Appears
194. Holy War in the Galaxy! Sailor Wars Legend
195. Princess Kakyuu Burns Out! Galaxia on Earth
196. Countdown to the Destruction of the Galaxy! Sailor Senshi's Final Battle
197. Ruler of the Galaxy, Galaxia's Threat
198. Disappearing Stars! The End of Uranus and Neptune
199. The Light of Hope! Final Battle for the Galaxy
200. Usagi's Love! The Moonlight Lights up the Galaxy
If anyone has any questions why I chose a particular episode, or why I didn't choose a particular episode, feel free to ask