Taking a pole here. What's the best costume you have ever worn or seen on someone else? To get it started here are a couple from my own past:
In first grade my parents thought it would be great if I went as Little Orphan Annie. I wore tights, buckeled shoes, a wig and a dress. FIRST GRADE! First place too so I won't hold it against them.
One year when my mom was single she put on a full body, flesh colored leotard and a 4 foot blond wig. Nothing else. She was Lady Godiva.