My most memorable trade(s) was when I was doing a giveaway on New Years. It wasn't quite about what I was trading or what I was getting, but the fact that I was outside trading while watching fireworks was pretty cool. It's a memory that's sticks out in my mind. Well, I find every giveaway fun to experience, but the fireworks were icing to the cake.
My least memorable trade was on another forum. The Crown shiny beasts were distributed and I hunted for one... I finally thought I made a good deal, but the Shiny Suicune turned out to be a hack. I was pretty angry, haha.