My Little Pony: Friendship is Magic - Image #766,028

I'm gonna say spike. And I don't just mean as a whole, but also proportionately. He had a whopping 3 episodes centered around him this season.

In "Power Ponies", he learns he really can contribute in a meaningful way
In "Inspiration Manifestation", he is forced to confront a dear friend who is going over the deep end
In "Equestria Games", he learns humility.

I'd probably say have to say Spike. His episodes did seem to have more of a profound effect on him compared to the others.

Maybe Fluttershy would be a close second, as episodes like Filli Vanilli had some good lessons for her, and her Key episode actually seemed well put together.

The other Mane Six never really had to learn anything THAT important, just them going through motions as the audience learned the lesson. Even their key episodes seemed like reminders of what they already knew, or straight up setbacks of them forgetting said knowledge only to re-learn it.(cough Rainbow Falls cough)