The best? Hard to say. One that always crosses my mind was when I was crossplaying as Chii, I was sitting on a bench outside of Dunkin Donuts and an elderly lady came up to me and said "I like your dress.
"..... I said thanks, and she sat down, and I started talking with her. She said she was always fascinated over the costumes she sees every year, and she comes to the mall area every year just for that reason. It was a sweet moment for me.
Another was when I trapped a gameshow host and she said into the mic "WAIT, YOU'RE A GUY?"..... then later, my team proceeded to win the event.
Oh, and at a "Name that tune" event, that one awkward song no one knows and being the person to say "Bible Black"..... yep, was a good moment.