Gemstone
Well, that entirely depends on what you want from the series. With the number of episodes growing, it becomes a question of whether you want a consistent story, world-building, or character establishing.
For example, Story based episodes would probably consist of the following:
1 & 2. Frienship is Magic is essential since it sets up the entire series.
3. Bridle Gossip introduces Zecora, who is a key character in several episodes
4. Call of the Cutie is needed to set up the explination of Cutie Marks and why they’re so important, which becomes a key concept in Season 5.
5 & 6. Return of Harmony introduces Discord
7. Lesson Zero changes the status quo by having ALL of the Mane 6 writing letters to Celestia
8. Luna Eclipsed introduces Luna and explains the show’s verision of Halloween
9 & 10: Canterlot Wedding introduces Changelings, Cadence, Shining Armor, and the magic of Love.
11 & 12. Crystal Empire and Sombra are both introduced, and it sets up Twilight’s potential Ascension
13. Magical Mystery Cure features Twilight’s Ascension
14 & 15: Princess Twilight introduces the Tree of Harmony, the Box, and has the Mane 6 give up their elements
16-20: These episodes are the Mane characters getting their Friendship Tokens. Rarity Takes Manehatten, Rainbow Falls, Pinkie Pride, Being Breezies, and Leap of Faith. One big problem is that these feature a lot of references to episodes not on the list, ESPECIALLY Pinkie Pride.
21 & 22: Twilight’s Kingdom has Twi get her Friendship Token, discover her role as the Princess of Friendship, fully establishes Discord as a good guy, AND features the destruction of the Golden Oaks Library which is then replaced by the Castle of Friendship. Kinda have to include this one for anything after Season 4.
23 & 24: The Cutie Map introduces the map and Starlight Glimmer
25 & 26: Cutie Re-Mark sets Starlight as a permanent part of the mane cast.
Few changes may apply, but this is pretty much the order to watch if you just want to follow the progression of the series as a whole.
For example, Story based episodes would probably consist of the following:
1 & 2. Frienship is Magic is essential since it sets up the entire series.
3. Bridle Gossip introduces Zecora, who is a key character in several episodes
4. Call of the Cutie is needed to set up the explination of Cutie Marks and why they’re so important, which becomes a key concept in Season 5.
5 & 6. Return of Harmony introduces Discord
7. Lesson Zero changes the status quo by having ALL of the Mane 6 writing letters to Celestia
8. Luna Eclipsed introduces Luna and explains the show’s verision of Halloween
9 & 10: Canterlot Wedding introduces Changelings, Cadence, Shining Armor, and the magic of Love.
11 & 12. Crystal Empire and Sombra are both introduced, and it sets up Twilight’s potential Ascension
13. Magical Mystery Cure features Twilight’s Ascension
14 & 15: Princess Twilight introduces the Tree of Harmony, the Box, and has the Mane 6 give up their elements
16-20: These episodes are the Mane characters getting their Friendship Tokens. Rarity Takes Manehatten, Rainbow Falls, Pinkie Pride, Being Breezies, and Leap of Faith. One big problem is that these feature a lot of references to episodes not on the list, ESPECIALLY Pinkie Pride.
21 & 22: Twilight’s Kingdom has Twi get her Friendship Token, discover her role as the Princess of Friendship, fully establishes Discord as a good guy, AND features the destruction of the Golden Oaks Library which is then replaced by the Castle of Friendship. Kinda have to include this one for anything after Season 4.
23 & 24: The Cutie Map introduces the map and Starlight Glimmer
25 & 26: Cutie Re-Mark sets Starlight as a permanent part of the mane cast.
Few changes may apply, but this is pretty much the order to watch if you just want to follow the progression of the series as a whole.