For Twilight, it was originally going to be Lesson Zero, because her "insane" personality was very very humorous and made it a very enjoyable episode to watch. But, I have to say A Canterlot Wedding revealed more about her than any episode ever could (besides the pilot, of course), and it just has everything she needed as a character in it. Plus, she was in a lot of emotional moments in this episode, plus she finally got her own song. Honorable mentions: Lesson Zero, Friendship is Magic Part 1, Winter Wrap Up, It's About Time
For Fluttershy, Putting Your Hoof down takes the cake for me. Besides being an outrageously hilarious episode, it completely changed her character, and she learned to be assertive, but having it taken a bit too far. The rhymes were actually NOT lame, like some things like that are in typical cartoons nowadays. Overall, I really really liked it. For second place, it'd be either Dragonshy or Hurricane Fluttershy for the same reasons, even if they were somewhat out-of-character for her. But it was in a good way, though. My favorite ep where she WAS in-charcater, was probably A Bird in the Hoof. Honorable mentions: Hurricane Fluttershy, Dragonshy, A Bird in the Hoof
I think it's obvious that Pinkie Pie is one of the most interesting when it comes to characteristics of the mane 6. She's a comic relief character, so you'd expect she'd be popular among us Bronies. This is a toughie, but my favorite Pinkie episode is Baby Cakes, because not only does it show off the positivities of her character, it was a very intriguing and funny episode, with a touching ending. Not to mention a VERY good message. It was also one of my top fave's of season 2. A Friend in Deed is a close 2nd, because it shows how willing she is to make a simple friemd, and how much of a likeable pony that she is. Feeling Pinkie Keen was a very hilarious episode to watch, and revealed something very important about her. Honorable mentions: A Friend in Deed, Feeling Pinkie Keen, Party of One, MMMMystery on the Friendship Express,