In chapter 9, when Mare Do Well invites Rainbow Dash to join her during one of her nightly superhero patrols and they actually encounter a group of criminals. Mare Do Well proceeds to kick their collective asses. Arguably becomes even more awesome once Mare Do Well is revealed to have been Pinkie Pie all along.
Meta-example for the author; for nineteen whole chapters - and a large portion of the twentieth - she manages to keep Mare Do Well's identity secret, while providing Red Herrings to keep the viewers guessing, to the point where some readers eventually gave up on trying to figure out who it was. Managing to do that, and still have The Reveal be greatly surprising, is definitely awesome.
Mostly thanks to Viewers Are Morons in that AA did drop several glaring hints that, in hindsight, made Mare Do Well being Pinkie rather obvious. Also, bonus points for having the IDENTITY of MDW not being the point of the mystery, but rather Pinkie'sREASON(S) for donning the costume in the first place.