The Nightmare Moon two parter were my first episodes.
Here were my first impressions:
Twilight Sparkle: A loner, but I came to realize all she needed was just to know what friendship really was. She's my favourite mane 6 now
Rainbow Dash: Cocky, but I knew she'd be funny in her own way.
Fluttershy: Timid, but she's so cute
Pinkie Pie: You can say she's my second favourite character now because of her hilarity, but I didn't think that before. I thought she was too silly when I first watched the show.
Rarity: Really cares about looks, which I don't like, but I appreciated her generosity towards the dragon at the Everfree Forest.
Applejack: The perfect character. I knew she had flaws when I watched Applebuck Season, but the fact she has few flaws just makes me wonder...