Either the episode with Cheese Sandwich as a background pony, or maybe the episode with Daring Do as a prisoner of the cultist.
I'm ok with any episode as long as it's not Griffon The Brush Off, Owl's Well That Ends Well, Just For Sidekicks, It Ain't Easy Being Breezies, Somepony To Watch Over Me or Trade Ya.
You may dislike her, that's fine(I hate her too, seriously), but don't you dare to hate those ep., since Rairty learned something there Yep.And why red anyway?
Mare Do Well: I AM THE TERROR THAT TROTS IN THE NIGHT! I AM THE POPCORN PELLET THAT GETS STUCK IN YOUR TEETH! I AM THE LETTUCE IN THE FRIDGE OF JUSTICE! I AM MARE DO WELL! Now, where's my transportation? *Airship McHoof crash-lands a plane nears Mare Do Well* Airship McHoof: Sorry about being late, Miss M-D-W! The Wonderbolts were doing flying practice, so the air was a bit crowded today! Mare Do Well: Never mind that, let's go! There's evil afoot!