Which main character has been "most improved"?

[heavens-champion 1,907]

Rarity. I can't remember the last time she complained about getting dirty.

[Megas 28,111]

Rarity definitely, she had an uphill battle considering the basis of her character is often some of the most disliked among girl's shows

[Here No Longer 5,286]

Most Improved- Probably Rarity. All of them (aside from Twilight and Pinkie) have improved to some degree, but Rarity has become a more intricate character with more behind her than "eww dirty!" Sure a lot of this character development happened in the first two seasons, but even after that she's just been getting better.

 

Least Improved (even though the topic doesn't ask)- Definitely Pinkie. Twilight has developed more maturity, even with how I don't entirely approve of the direction they took her personality (making her less relatable, and making her closer to Mary Sue at some times). Pinkie, however, has gotten progressively more ANNOYING in recent seasons. 

 

 

[Dark Qiviut 22,588]

• Spike: He always had the short end of the stick, and with him being the youngest (and only male) of the Mane 8, the Spikeabuse hurt, especially in his episodes. Following AF, he improved drastically. While everyone minus one suffered in S6, he was the best written by far.
• Fluttershy: After breaking out in S1, DHX stuck her in a major lull from Luna Eclipsed to around Trade Ya! S5 brought her out of it, and S6 and 7 improved on that. S8 was a bit of a step back, thanks to Fake It.
• Rarity: When S1 began, she got off to the wrong foot, exemplifying at times the trepidation for her (i.e., Boast Busters, Bridle Gossip). Not till Suited where she showed signs of relatability.

And if we're going by "secondary" mane characters, count the CMCs, too. S1 was, for the most part, not kind for them. In almost all their appearances, they were one-dimensional child characters. S2 brought forth a refreshing direction for them all.

[Midnight Solace 5,134]

I guess Glim Glam or Baconmane. It would suck if they were still baddies.