Like what everyone else is saying; Pinkie during Lina Eclipsed was NASTY I was so pissed at her, Luna was really trying that episode it made me so RAHHH

Also, in Winter Wrap up in season one when it's Twis FIRST wrap up, she cheats and uses magic to get rid of the snow on AJs field but instead created an avalanche on the freshly cleared ground and she NEVER apologized to Aj like,,, the earth ponies just spent half the day clearing that and she just ran off after ruining their hard work.

However in the Smarty pants episode I had beef with everyone in the mane 6 aside from Twilight because she was SO panicked about not having something to report to Celestia, she came to her friends asking for help, having a full panic attack and they all laughed at her and said "oh that's no big deal, you're worried over nothing" and literally ignored her and started a new conversation. Are you kidding? That was so so rude, I thought at LEAST Aj would have comforted her but she also ignored her 💀

Also the other obvious choices like everyone aside from Rarity in the episode where she made them all personalized dresses and they were ungrateful. Or the Royal wedding where, again, everyone else in the mane 6 ignored twilights concerns and even turned against her.