How would you rank them from best to worst?
For me, from worst to best:
6: Season 6
We're only seven episodes in but the season seems to be lacking the entertainment of previous seasons. None of the episodes are particularly bad IMO, but none of them are great either, except Gauntlet of Fire which was due in large part to Ember, who is a dragon rather than a pony. Even the Rainbow Dash episode, which I'm usually biased towards, and seeing her achieving her dream no less, seemed just "all right" to me. Furthermore none of the upcoming episodes except Hearth's Warming Tail sound interesting to me, possible exception of Flutter Brutter on account of being a Fluttershy episode and the new character of her brother (and aside from it it doesn't sound like we're getting much Fluttershy this season, even though it's her turn:
- Season 1 - Twilight Sparkle, who appeared in every episode even if it didn't revolve around her
- Season 2 - Rainbow Dash, who got two episodes in a row, one to deflating her ego and another to getting a pet, plus an episode where she gets into reading
- Season 3 - Applejack, who appeared in every episode, two of which revolved around her and we get more insight to her family
- Season 4 - Pinkie Pie, who got three episodes, one of which introduces us to her sister and another of which suggests she and Applejack are cousins
- Season 5 - Rarity, who got three episodes in a row and opened up a shop in Canterlot
This is quite a rarity, where I think a Spike episode is awesome and a Rainbow Dash episode is simply all right
Also a little disappointed that Starlight isn't one of the Mane 6, but rather an extra Spike, Luna, or CMC, but I can live with this because I know most of the fans don't want her to be one of the Mane 6.
5: Season 3
There were some really good episodes (The Crystal Empire being high up my list for certain reasons, also Magic Duel, followed by two episodes that show the better side of Rainbow Dash's personality and make progress for both her relationship with Scootaloo and her quest to become a Wonderbolt, then Discord's redemption in Keep Calm and Flutter On, and finally Magical Mystery Cure, which was a good musical and such. This is lower than Season 1 because a lot of the other episodes were nothing as impressive and aside from the first and last episodes, there weren't many songs. And no Rarity episodes, taking place at the same time as a comic which treated her quite badly, between the two I felt just awful for her.
4: Season 1
Season 1 was the least extreme season to the point it couldn't have true villains that weren't corrupted aside from dragons, Hydra, etc. but it was what first created the fandom in the first place, thus setting the stage for more epic seasons and dark villains. There were plenty classic or enjoyable episodes there which seemed to define the series and characters at first, and you're more familiar with them. (it's also the strongest season with, ahem, one of my favorite elements)
3: Season 2
Season 2 was the first season created with Hasbro and DHX well aware of the fandom, so they put in more appeal to us and allowed for more epic scenes, beginning with Discord the villain who was fun to have around, and finishing with surprise villain Queen Chrysalis, first Knight of Cerebus in the show, in what was my favorite episode until it was overtaken by Twilight's Kingdom. The season was full of fun episodes and good songs. And there was an adorable Fluttershy episode, as well as my favorite Slice of Life episode.
2: Season 4
Season 4 was like Season 2, but better. With the Keys of Friendship arc, some awesome episodes, and some excellent songs. The first episode was underwhelming with choice of villain but the flashbacks to Nightmare Moon and Discord cancelled this out. The Equestria Games they had been building up to was a letdown, but they made up for it with the following episode, the most epic two-parter with Tirek, a reboot of the first G1 villain with a Dragonball Z style battle that is sure to convince any skeptics of the show's popularity. My personal favorite episode. Oh, and another adorable Fluttershy episode.
1: Season 5
Season 5 started out disappointing and I almost gave up on it, but was convinced not to, and it's a good thing too because there were several sensational moves this season.
- Rainbow Dash gets her own song
- An episode revolving around Discord
- Gilda's redemption (plus a death that even those who thinks Sombra survived can't deny)
- An episode focusing on background ponies with several things specifically thrown in for the fans, including the return of Steven Magnet and canonizing both of BonBon's names
- Going back to Moondancer from the first episode
- Another Luna episode also with lots of stuff thrown in for the fans including Rainbow Dash dreaming of fighting changelings
- Three Rarity episodes in a row to make up for Season 3 (even if two of them weren't as interesting to me)
- One of which was a long-awaited Rarity/Rainbow Dash episode, even if I liked MMMystery on the Friendship Express better
- The Cutie Mark Crusaders get their cutie marks (right after Diamond Tiara's redemption)
Pinkie Pie blushing for the first timeNevermind that, everypony who found that adorable left when Twilight became an alicorn, yet another fandom I joined too late and unless I hear otherwise this is the last time I will mention it (this is what I was getting at with Rainbow Dash during her Fluttershy impression)
- Episode with alternate futures showing in detail King Sombra, Queen Chrysalis, and Nightmare Moon in power, thus giving us Chrysalis and Sombra returns without saying they survived (plus a few seconds of Tirek), even saying Chrysalis's name for the first time while giving more insight to how evil Sombra is (though I'd have liked it better if he had lines)
There were some forgettable episodes, and a controversial one written by someone who I heard is anti-brony, but the others balance it out. The earlier disappointment of The Cutie Map was cancelled out by Starlight's redemption at the end. My least favorite episode the season was Appleoosa's Most Wanted, but the reason I didn't list it as forgettable was because it helped set the stage for the Cutie Mark Crusaders getting their cutie marks.
For best season overall: