The Episode
Oh god the first time i watched Sonic Rainboom I knew i had found my "best pony".
Backstory
I had started watching the series (as i do with every tv show) chronologically and i had already known about Rainbow Dash beforehand considering when i first got into it she was by far (and still is) one of the more popular ponies out of the mane 6. Now she was the first pony to make me laugh when she crashed into Twilight and said "oops hehe sorry, here let me help you." and when she proceeded to absolutely douse Twilight i lost it. After that i decided to direct my attention to her whenever she was on screen, when i saw she was the element of loyalty i freaked out because Loyalty and Generosity are by far the most important virtues to me. So here i was after the season 1 premiere with a purple unicorn who was ok i guess, orange pony with no powers who was a country bumpkin, another pony with no powers who was really pink but good at singing, a yellow flying pony who was shy but great with animals, a white unicorn who was so fucking prissy, and then you had this rainbow colored flying pony who was hilarious, loyal, and just so fucking confident and awesome!
I started watching the next few episodes and my heart completely melted when she went "so awesome" and once again laughed my ass off when she was on screen with the orange nopowers pony. Then when she started having some development with the pink pony i started loving that pink one and when she told Gilda to fuck off after being a bitch i was like "awwww yiss!" Then of course she showed up Trixie in Boast Busters and kicked a fucking dragon in dragonshy! So by this point i picked up on the terms earth, unicorn, and pegasus ponies and still loved Dashie but my perspective changed on the other mane 6 as well, we had this white unicorn who wasn't so prissy as a thought and actually very clever and funny, you had an adorable yellow pegasus pony who cared about everyone, you had an amazing and energetic pink party pony who was an amazing singer, you had a very hardworking and honest orange earth pony who had a mean hat, and you had a very smart and levelheaded purple unicorn. Then came the episode...the one that would change the way i watched MLP forever: Sonic. Rainboom. I mean this is the episode that literally made me see Dashie as more than a regular pegasus pony and understand the magnitude of her awesomeness is always 20% cooler than anyone elses in the room at any given time!
Since then I've been with the show for a little while now and have to saw Dashie will always have a special place in my heart even when i'm 30 or 40...And that children is the story of how i picked my best pony.