Shes loyal, but ambitious and hard to please. She most definately has the most to learn, in my opinion.

^ That is very true. It's a toss-up among her, Twilight, and Rarity. Then maybe Pinkie (who seems to have learned fewer lesson than others so far, and hasn't had a lot of focus in season 2 yet), Applejack, & Fluttershy. Naturally, they all have things they need to learn about being a friend, as they grow.

All the ponies have imperfections.
I chose Rainbow because she still seems to have a problem with pride and selfishness. She seems to control that most of the time, but it's still a visible problem.
I would have chosen Rarity in the past, but it seems like she's been through a lot that has changed her.

She's hard to please, but loyal. Like Fairstepshaven and BillyTheShark pointed out, she's ambitious, sometimes selfish, and proud. I agree.

Twilight because now she actually can make friends. However, people do have a point about Rainbow Dash she is pretty conceited.

I think Dash has had more eye opening moments with the others. She learned to not let her competitiveness get in the way of friendship, and greatly strengthened her relationship with AJ. She learned to be more sensitive and understanding with Fluttershy and they also seem very close.

Some episodes I'd like to see in season 3 would be two of the main ponies resolving some sort of issue they have with each other and becoming much closer because of it. I'd like to see a Dash/Twi episode and Dash/Rarity.

Overall I think Rainbow Dash had the most to learn and has come farther than the others in regards to where they all started.

Hmmm maybe